Method of assembling the optical module implementing mach-zehnder modulator

ABSTRACT

An optical module and a method of assembling the optical module are disclosed. The optical module comprises a laser unit, a modulator unit, and a detector unit mounted on respective thermo-electric coolers (TECs). The modulator unit, which is arranged on an optical axis of the first output port from which a modulated beam is output, modulates the continuous wave (CW) beam output from the laser unit. On the other hand, the laser unit and the detector unit are arranged on another optical axis of the second output port from which another CW beam is output. The method of assembling the optical module first aligns one of the first combination of the laser unit and the modulator unit with the first output port and the second combination of the laser unit and the detector unit, and then aligns another of the first combination and the second combination.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part application of U.S.patent application Ser. No. 15/510,607, filed Mar. 10, 2017, which is aNational Stage application of PCT/JP2015/005433, filed Oct. 28, 2015,which claims priority to Japanese Patent Application No. 2015-008963,filed Jan. 20, 2015, Japanese Patent Application No. 2015-006130, filedJan. 15, 2015, Japanese Patent Application No. 2014-251138, filed Dec.11, 2014, Japanese Patent Application No. 2014-236635, filed Nov. 21,2014, and Japanese Patent Application No. 2014-219585, filed Oct. 28,2014; this present application also claims priority to Japanese PatentApplication No. 2016-052615, filed Mar. 16, 2016; this presentapplication is also related to the following commonly-assigned U.S.patent application Ser. No. 14/831,492, filed Aug. 20, 2015, which is acontinuation-in-part application of U.S. patent application Ser. No.14/707,468, filed May 8, 2015, now U.S. Pat. No. 9,490,900; all of whichare incorporated herein by references.

TECHNICAL FIELD

The present invention relates to an optical module that installs anoptical source including a semiconductor laser diode (LD), an opticalmodulator, and a wavelength detector; and the invention further relatesto a method of assembling the optical module.

BACKGROUND

An optical module that installs a wavelength tunable semiconductor laserdiode (t-LD) and an optical modulator that modulates CW light emittedfrom the t-LD has been well known in the field. A Japanese patentapplication laid open No. 2009-146992 has disclosed such an opticalmodule. The CW light output from the t-LD optically couples with theoptical modulator via optical fibers. However, an optical fiber when itis bent with a large curvature causes a bent loss. Accordingly, when anoptical transceiver with limited sizes in a housing thereof installs at-LD and an optical modulator, techniques to compensate the bent losscaused in inner fibers is needed.

SUMMARY

An aspect of the present invention relates to a process of assembling anoptical module that includes an optical modulator, a beam combiner, andan output port. The optical modulator has a type of Mach-Zehndermodulator that provides a first port and a second port. The first portoutputs a first beam, while, the second port output a second beam. Thefirst port is coupled with the output port through a first path thatimplements a first lens and a second lens thereon. The second port isalso coupled with the output port through a second path that implementsa first lens and a second lens thereon. The beam combiner combines thefirst beam with the second beam, and generating a combined beam to theoutput port. The first lenses on the first path and the second path areplaced closer to the first port and the second port compared with thesecond lenses on the first path and the second path, respectively. Themethod comprises steps of:

(1) moving the first lens on the first path and the first lens on thesecond path closer to the first port and the second port by a presetdistance from positions at which the first lenses on the first path andthe second path convert the first beam and the second beam eachdiffusively output from the first port and the second port intocollimated beams;(2) placing the second lenses at positions where the first beam and thesecond beam output from the first lenses couple with the output portwith respective maximum efficiencies;(3) determining one of the second lenses that shows the maximum couplingefficiency greater than the maximum coupling efficiency shown by anotherof the second lenses; and(4) equalizing the maximum coupling efficiencies by moving the one ofthe second lenses that shows the greater maximum coupling efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other purposes, aspects and advantages will be betterunderstood from the following detailed description of a preferredembodiment of the invention with reference to the drawings, in which:

FIG. 1 is a perspective view of an optical module according to anembodiment of the present invention;

FIG. 2 shows an inside of the optical module shown in FIG. 1;

FIG. 3 shows a cross section of a wavelength tunable LD implementedwithin the optical module shown in FIG. 1;

FIG. 4 is a plan view of a laser unit;

FIG. 5 is a plan view of an optical modulator;

FIG. 6 is a plan view of a detector unit;

FIG. 7 is a plan view of a modulator unit;

FIG. 8 is an exploded view of the modulator unit;

FIG. 9 is a plan view of the base of the modulator unit, where the basemounts the optical modulator through a carrier;

FIG. 10 is a plan view of a terminator unit mounted on the base shown inFIG. 9;

FIG. 11 is a plan view of a bias unit mounted on the base shown in FIG.9;

FIG. 12 is a plan view of an input unit to guide the first CW lightemitted from the laser unit shown in FIG. 4 into the optical modulatorshown in FIG. 2;

FIG. 13A schematically shows a ray tracing of the one lens system, andFIG. 13B shows a ray tracing of the two lens system;

FIG. 14A to FIG. 14F show coupling tolerances of the one lens system(FIGS. 14A and 14B), coupling tolerances of the first lens in thetwo-lens system (FIGS. 14C and 14D), and coupling tolerances of thesecond lens in the two-lens system (FIG. 14E and FIG. 14F);

FIG. 15 is a plan view of a joint unit;

FIG. 16 is a plan view of an output unit;

FIG. 17 shows a cross section of the optical module taken along theoptical axis extended from the second output port;

FIG. 18 shows a cross section of the optical module taken along theoptical axis extended from the first output port;

FIG. 19 is a plan view of the arrangement along the wiring substrates,the laser unit, and the detector unit;

FIG. 20 is a plan view of the arrangement around the laser unitincluding two wiring substrates;

FIG. 21 is a perspective view of the laser unit and two wiringsubstrates;

FIG. 22 shows a flow chart of a process of assembling the optical moduleshown in FIG. 1;

FIG. 23 shows a process of assembling the t-LD on the LD carrier;

FIG. 24 shows a process of assembling the optical modulator, the inputunit, the joint unit, two bias units, two terminator units, and two PDsub-mount on the base of the modulator unit;

FIG. 25 shows a process of aligning the joint unit and the input unit onthe base of the modulator unit with the laser unit;

FIG. 26 shows a process of assembling the output unit with the opticalmodulator;

FIG. 27 is a plan view showing a process of installing the TECs, thewiring substrates, and the VOA substrate into the housing of the opticalmodulator;

FIG. 28 is a plan view showing a process of mounting the LD carrier andthe lens carriers on the first TEC, the base of the modulator unit,which mounts the input unit, the joint unit, the output unit, the biasunits, the terminator units, and the PD sub-mounts thereon;

FIG. 29 shows a process of aligning the detector unit with the laserunit;

FIG. 30 shows a process of aligning the first collimating lens in aposition at which the output beam of the collimating lens becomes acollimated beam;

FIG. 31 shows a process of shifting the optical axis of the laser unitso as to be aligned with the optical axis of the optical modulator;

FIG. 32 shows a process of aligning the beam splitter in the input unit;

FIG. 33 shows a process of aligning the first lens and the second lenswith the input port of the optical modulator;

FIG. 34 magnifies a portion of the output unit in the modulator unit;

FIG. 35A schematically illustrates a process for aligning the first lenswith the optical modulator, FIG. 35B schematically illustrates a processof finding the initial position of the second lens, and FIG. 35Cschematically illustrates a process of using a dummy port; and

FIG. 36A illustrates a process of aligning the second lens in thesubsidiary path with the dummy port, and FIG. 36B illustrates a processof aligning the second lens in the primary path with the dummy port.

DETAILED DESCRIPTION

Next, some preferred embodiments will be described as referring todrawings. In the description of the drawings, numerals or symbols samewith or similar to each other will refer to elements similar to or samewith each other without overlapping explanations.

First Embodiment

FIG. 1 is a perspective view of an optical module according to anembodiment of the present invention and FIG. 2 shows an inside of theoptical module shown in FIG. 1. The optical module 1 of the presentembodiment may be implemented within an optical transceiver applicableto the optical coherent system. The optical coherent system utilizes thephase of light, in addition to the magnitude thereof, as one bitinformation. When the optical signals corresponding to the phasecomponents of 0° and 90°, that is, when the coherent system multiplexesthe optical signals each having the phase components of 0° and 90°, thesystem may transmit two-bits information at the same time.

The optical module 1 includes a laser unit 100, a modulator unit 200,and a detector unit 300 within a housing partitioned by a front wall 2A,a rear wall 2B, and two side walls, 2C and 2D, connecting the front wall2A to the rear wall 2B. The laser unit 100 optically couples with boththe modulator unit 200 and the detector unit 300. Specifically, theoptical module 1 outputs a modulation signal D1 from the first outputport 3 a, where the modulation signal D1 is obtained by modulating firstcontinuous wave (CW) beam L1 output from a wavelength tunable laserdiode (t-LD) 10 implemented within the laser unit 100 by an opticalmodulator 20 installed in the modulator unit 200. Concurrently with thefirst modulation signal D1, the optical module 1 may output anotheroptical signal D2 from the second output port 3 b, where the opticalsignal D2 is originated from the other CW beam L2 output from the t-LD10 to the detector unit 300 and divided in the detector unit 300. Thefirst CW beam L1, which is output from the t-LD 10 substantially inparallel to the optical axes of the output ports, 3 a and 3 b, towardthe rear wall 3B, enters the optical modulator 20 along the rear wall 2Bbent by substantially 90°. The other CW beam L2, which is emitted fromthe t-LD 10 substantially in parallel to the optical axes of the outputports, 3 a and 3 b, toward the front wall 2A.

The optical module 1 of the present embodiment has a feature that theoptical module 1 mounts the laser unit 100, the modulator unit 200, andthe detector unit 300 on respective thermo-electric coolers (TECs)implemented in the housing independently. Moreover, the optical module 1provides radio-frequency (RF) terminals 4 only in the rear wall 2B, andDC terminals, 5 a and 5 b, in the respective side walls, 2C and 2D.Because the RF terminals 4 and the DC terminals, 5 a and 5 b, areindependent in respective walls; the electrical control of the opticalmodule 1 may be simplified and stabilized.

The modulator unit 200, as described above, modulates the first CW beamL1 in the phase thereof and outputs the phase-modulated optical signal.That is, the optical modulator 20 implemented within the modulator unit200 divides the first CW beam L1 into four beams, and modulates thesefour beams independently by four modulation signals provided through theRF terminals 4, where two of four modulated signals output from theoptical modulator 20 have phase components different by 90° from therest of two modulated signals. The former two modulated signals areoften called as I-components (In-phase component), while, the latter twomodulated signals are called as Q-components (Quadrature component). Oneof I-components and one of Q-components are further modulated by thepolarization thereof. That is, one of the I-components and one of theQ-components are rotated in the polarization thereof and multiplexedwith the other of the I-components and the other of the Q-components.The optical module 1 may output the modulated signal D1, whichmultiplexes four optical signals, from the first output port 3 a as thephase-polarization modulated signals, which is often called as the dualpolarization quadrature phase shift keying (DP-QPSK). The optical module1 may further output another optical signal D2, which is obtained bydividing the second CW beam L2 output from the t-LD 10 by the detectorunit 300. One of the divided CW beam is used for determining thewavelength of the CW beam L2, and the rest is output from the secondoutput port 3 b as the output CW beam D2.

Next, details of the respective units, 100 to 300, will be described.

Tunable Laser Diode (t-LD)

FIG. 3 shows a cross section of a wavelength tunable LD implementedwithin the optical module shown in FIG. 1. The t-LD 10 includes twosemiconductor optical amplifiers SOAs, 10 a and 10 d, and a sampledgrating distributed feedback (SG-DFB) 10 b and a sampled gratingdistributed Bragg reflector (SG-DBR) 10 c, where the latter two regions,10 b and 10 c, may determine the emission wavelength of the t-LD 10.These four regions are arranged along an optical axis of the t-LD 10.The present t-LD 10 provides one facet 10A in one of the SOA 10 a totransmit the first CW beam L1 and another facet 10B in the other SOA 10d to transmit the second CW beam L2.

The SG-DFB 10 b includes a sampled grating (SG) 18, where the sampledgrating 18 is featured by regions each including a plurality of gratingsand separated by spaces without any gratings. The gratings in respectiveregions have a constant pitch and the spaces have a constant lengthalong the optical axis. When the spaces have various lengths, thesampled grating may be called as the chirped-sampled grating. TheSG-DFBs 10 b includes gain regions, 12 a to 12 c, including the SG 18,and modulation regions, 13 a and 13 b, also including the SG 18. Thegain regions, 12 a to 12 c, may be provided with carriers throughelectrodes 14 a on a top surface of the device. On the other hand, themodulation regions, 13 a and 13 b, provides heaters, 15 a and 15 b, inthe top surface thereof. A combination of the gain regions, 12 a to 12c, and the modulation regions, 13 a and 13 b, the SG-DFB 10 b may showthe optical gain spectrum having a plurality of gain peaks reflectingthe SG 18 in the SG-DFB 10 b. Providing power to the heaters, 15 a and15 b, that is, heating up or cooling down temperatures of the waveguidelayers 19 b beneath the heaters, 15 a and 15 b, optical characteristicsof the modulation regions, 13 a and 13 b, may be modified, that is,wavelengths of the gain peaks inherently attributed to the SG-DFB 10 bmay be changed.

The CSG-DBR 10 c of the present embodiment provides three sections, 16 ato 16 c, each having heaters, 17 a to 17 c, operable independently.Because the CSG-DBR 10 c does not includes any gain regions, the CSG-DBR10 c inherently show reflection spectrum having a plurality ofreflection peaks. Powering the heaters, 17 a to 17 c, to modifytemperatures of the waveguide 19 b beneath the heaters, 17 a to 17 c,the reflection peaks in the spectrum of the CSG-DBR 10 c may be changedin the wavelengths and intervals thereof. At least one of the sections,16 a to 16 c, has physical features distinguishable from those of therest sections. In the present t-LD 10, the section, 16 a to 16 c,provides optical lengths different from others. That is, the spaceswithout diffraction gratings have respective optical lengths differentfrom others, which are called as the chirped-sampled diffraction Braggreflector (CSG-DBR). The reason why the t-LD 10 of the presentembodiment provides the CSG-DBR, not the SG-DBR, is that a range wherethe reflection peaks appears may be widened by modifying thetemperatures of the waveguides in respective regions independently.Adjusting the power supplied to the heaters, 15 a and 15 b, in theSG-DFB 10 b and the heaters, 17 a to 17 c, in the CSG-DBR 10 c, one ofthe gain peaks attributed to the SG-DFB 10 b matches with one of thereflection peaks attributed to the CSG-DBR 10 c. Then, the SG-DFB 10 band the CSG-DBR 10 c may form a cavity for the t-LD 10 and the t-LD mayoscillate at the matched wavelength. This matched wavelength is optionalby adjusting the power supplied to the heaters, 15 a and 15 b, and 17 ato 17 c.

The first and second SOAs, 10 a and 10 d, may amplify an optical beamgenerated by the gain regions, 12 a to 12 c, and determined in thewavelength thereof by the optical coupling of the SG-DFB 10 b with theCSG-DBR 10 c. The optical gain of the SOAs, 10 a and 10 d, may bevariable by injecting carries into the active layer 19 a through theelectrode 14 d in the first SOA 10 a, and carries into the other activelayer 19 a through the electrode 14 e in the second OSA 10 d. Thus, theamplitude of the first and second CW beam, L1 and L2, are variable. Thewaveguide 19 b in the modulation regions, 13 a and 13 b, in the SG-DFB10 b and that in the CSG-DBR 10 c may be made of semiconductor materialwith energy band gap greater than that of the active layer 10 a in theSOAs, 10 a and 10 b, and the gain regions, 12 a to 12 c, in the SG-DFB10 b to make the waveguide 19 b substantially in transparent for theoptical beam subject to the t-LD 10.

FIG. 4 is a plan view of a laser unit 100. The laser unit 100 includes afirst thermo-electric cooler (TEC) 11 that mounts two collimatinglenses, 110 a and 110 b, and the t-LD 10 through a base 100 a.Specifically, the first TEC 11, which includes a top plate 11 a, abottom plate 11 b, and a plurality of thermo-electric convertingelements, typically Peltier elements, may cause a temperature differencebetween two plates, 11 a and 11 b, depending on a magnitude and adirection of a current flowing in the Peltier elements. The bottom plate11 b has a size wider than that of the top plate 11 a. That is, thebottom plate 11 b has a portion exposed from the top plate 11 a, and twoposts, 11 c and 11 d, to supply the current to the Peltier elements onthe bottom plate 11 b. The temperature of the top plate 11 a may besensed by a thermistor 11 f mounted on the top plate 11 a.

The base 100 a, which has a size substantially same with that of the topplate 11 a of the first TEC 11, may be made of aluminum nitride (AlN)and mounts two collimating lenses, 110 a and 110 b, through respectivelens carriers, 110A and 110B, and the t-LD 10 and the thermistor 11 fthrough an LD carrier 100A. These carriers, 100A, 110A and 110B, may bealso made of AlN but the LD carrier 100A has a thickness greater thanrespective thicknesses of the lens carriers, 110A and 110B, to match thelevel of the optical axis of the t-LD 10 with those of the collimatinglenses, 110 a and 110 b. The LD carrier 100A provides interconnections100 b thereon to provide biases to the t-LD 10. The t-LD 10 is necessaryto be supplied with a bias to inject carriers into the gain regions, 12a to 12 c, power to the heaters, 15 a and 15 b, in the SG-DFB 10 b,power to the heaters, 17 a to 17 c, in the CSG-DBR 10 c, biases to theSOAs, 10 a to 10 d, to secure the optical gains therein, and somegrounds. The LD carrier 100A requires the interconnections 100 b tosupply these biases and power to the t-LD 10.

Optical Modulator

FIG. 5 is a plan view of an optical modulator. The optical modulator 20may include a plurality of modulation elements, for example four (4)Mach-Zehnder (MZ) elements, 51 to 54, are integrated on a semiconductorsubstrate made of indium phosphide (InP) in the present embodiment. Theoptical modulator 20 of the embodiment includes three 1:2 couplers, 50 ato 50 c, to distribute the CW beam L1 entering from the input port 24into four MZ elements, 51 to 54. Specifically, the CW beam L1 enteringthe input port 24 is bent substantially in a right angle along thewaveguide and evenly divided into two partial beams by the first 1:2coupler 50 a. The respective partial beams are further evenly divided bythe second and third 1:2 couplers, 50 b and 50 c, into four partialbeams, and the four partial beams enter the MZ elements, 51 to 54,respectively. Two 2:2 couplers, 50 d and 50 e, are provided indownstream of the MZ elements, 51 to 54, to multiplex the modulatedbeam.

The explanation below concentrates on the first MZ element 51. But,other MZ elements, 52 to 54, may operate in the same manner with thefirst MZ element 51.

The partial CW beam divided by the second 1:2 coupler 50 b and enteringthe MZ element 51 is further evenly divided into two portions by the 1:2coupler 51 a each heading the arm waveguides, 51 h and 51 i. In the armwaveguides, 51 h and 51 i, in particular, within the functional region51M providing the modulating electrodes, 51 e and 51 f, and the groundelectrode 51 g, the divided beam are modulated in the phases thereof.After passing the functional region 51M, the divided beam in the phasesthereof are further modulated, or offset in the offset electrodes, 51 jand 51 k. Finally, the divided beams are combined by the 1:2 coupler 51b to be output from the MZ element 51.

The operation of the functional region 51M and the offset electrodes, 51j and 51 k, will be described. The offset electrodes, 51 j and 51 k arestatically pre-biased such that the optical beams propagating in therespective arm waveguides, 51 h and 51 j, have a phase difference of pi(π). For instance, the optical beam propagating in the one arm waveguide51 h is delayed by pi (π) with respect to the beam propagating in theother arm waveguide 51 j. Then, one of the modulating electrodes 51 efor the arm waveguide 51 h is supplied with a bias to cause the phasedelay of pi (π) but the other modulation electrode 51 f is supplied witha bias causing no phase delay. The beam propagating in the arm waveguide51 h is caused by the phase delay of 27 r by the modulation electrode 51e and the offset electrode 51 j; but, the beam propagating in the otherarm waveguide 51 i shows no phase delay caused by the modulationelectrode 51 f and the offset electrode 51 k. Combining two opticalbeams each propagating in the arm waveguides, 51 h and 51 i; thecombined beam shows a phase delay of zero. The phase delay of 27 r isequal to the phase delay of 0.

On the other hand, when the modulation electrode 51 e is supplied with abias causing no phase delay for the beam propagating in the armwaveguide 51 h thereunder but the other modulation electrode 51 f issupplied with a bias causing the phase delay of pi (π); the beamcombined by the 2:1 coupler 51 b has the phase delay of pi (π) becausethe former beam propagating in the arm waveguide 51 h is delayed in thephase thereof by the static bias of the offset electrode 51 j. Thus, theoptical output of the MZ element 51 becomes CW beam whose phase ismodulated between 0 and pi (π) but the amplitude thereof is keptsubstantially constant. The amplitude of the optical output strictlychanges at the transitions of the phase. Referring to FIG. 5, themodulation signals provided to the modulation electrodes, 51 e to 54 eand 51 f to 54 f, are supplied from the pads, 41 to 44, which are formedin one edge of the optical modulator 20, through interconnections whichare terminated at pads, 45 a and 45 b, provided in respective sides ofthe optical modulator 20 in downstream of the modulation electrodes, 51e to 54 e and 51 f to 54 f. Also, the static biases supplied to theoffset electrodes, 51 j to 54 j and 51 k to 54 k, are provided from thepads, 46 a and 46 b, formed in respective sides of the optical modulator20.

The quadrature electrodes, 51 c to 54 e, in the function thereof will bedescribed. The optical modulator 20 of the embodiment includes four (4)MZ elements, 51 to 54. The two quadrature electrodes, 52 c and 54 c, aresupplied with static biases such that the phases of the beamspropagating thereunder cause a phase difference of π/2 with respect tothe other beams propagating in the waveguides under the quadratureelectrodes, 51 c and 53 c, which form the pairs with the respectivequadrature waveguides, 52 c and 54 c. Accordingly, even after combiningtwo optical beams each propagating in the waveguides under thequadrature electrodes, 51 c and 52 c, 53 c and 54 c, the optical beamsmay be independently extracted. The optical beams output from the MZelements, 51M and 52M, and those from the MZ elements, 53M and 54M, maybe multiplexed with respect to the phases, one of the optical beams,subject to the MZ elements, 51M and 53M, are called as the I-component(In-phase), and the other is called as the Q-component (Quadrature). Theoptical modulator 20 may output two optical signals, M2 b and M2 c, eachmodulated in the phase, from respective output ports, 22 a and 22 b, andother two optical signals, M2 a and M2 d, from respective monitor ports,25 a and 25 b.

In the optical modulator 20 thus described, the modulation of theoptical beams may be carried out by varying refractive index of thewaveguide made of semiconductor materials in the functional regions, 51Mto 54M. A semiconductor material shows a large electro-optical couplingefficiency, which is called as the Kerr effect, for the optical beamwhose wavelength is slightly longer than a bandgap wavelength of thesemiconductor material, which corresponds to the bandgap energy of thematerial. A larger Kerr efficiency means that a modulation signal with asmaller amplitude may cause the substantial modulation in opticalcharacteristics of the semiconductor material. However, the bandgapwavelength of the semiconductor material has substantial temperaturedependence, which results in a large variation of the modulationcharacteristic of the optical modulator 20. The present optical module 1mounts the optical modulator 20 on the second TEC 20 to compensate thetemperature dependence of the modulation performance thereof.

Wavelength Detector

FIG. 6 is a plan view of a detector unit 300. The detector unit 300includes a first beam splitter (BS) 32 a, an etalon filter 33, a firstmonitor photodiode (m-PD) 34 a, a second BS 32 b, and a second m-PD 34b, where these components are mounted on a third TEC 31 through acarrier 300 a made of AlN, where the carrier 300 a will be referred asthe third carrier. Two m-PDs, 34 a and 34 b, are mounted on the carrier300 a through respective PD sub-mounts, 34A and 34B. The third TEC 31,similar to those first and second TEC, 11 and 21, provides a top plate31 a and a bottom plate 31 b. The bottom plate 31 b is wider than thetop plate 31 a and the carrier 300 a, and mounts two posts, 31 c and 31d, in an area exposed from the top plate 31 a and the carrier 300 a inorder to supply a current to Peltier elements mounted on the bottomplate 3 lb. The TEC 31 may compensate temperature characteristics of theetalon filter 33.

The etalon filter 33, which may be a parallel piped plate, shows aspecific transmittance, in particular, periodical transmittanceexhibiting strong wavelength dependence determined by a thickness of theparallel piped plate and refractive index of a material constituting theparallel piped plate.

The first BS 32 a and the second BS 32 b of the present detector unit300 have a type of slab made of material substantially transparent tothe second CW beam L2, typically, two BSs, 32 a and 32 b, may be a slabmade of silica glass. The first BS 32 a splits the second CW beam L2,which is output from the t-LD 10 and converted into a collimated beam bythe second collimating lens 110 b, into two beams. One of the split beamadvances to the etalon filter 33, while, the rest of the split beam goesto the second BS 32 b. The present embodiment of the detector unit 300sets a ratio of two beams to be 5:95, that is, about 5% of the second CWbeam LS enters the etalon filter 33, and the rest 95% goes to the secondBS 32 b. The former split beam transmitting through the etalon filter 33enters the second m-PD 34 b. The other split beam, which is bent by aright angle at the first BS 32 a, goes to the second BS 32 b and issplit thereby into two beams. One of the split beams passing through thesecond BS 32 b enters the first m-PD 34 a, and the other beam, which isreflected in a right angle by the second BS 32 b, is output from theoptical module 1 as the second output D2. The split ratio of the secondBS 32 b is set to be also 5:95. Accordingly, the output beam D2 has themagnitude of about 90% of that of the second CW beam L2 entering thedetector unit 300. The residual of the second CW beam L2 enters thefirst and second m-PDs, 34 a and 34 b, to determine the wavelength ofthe second CW beam L2.

The detector unit 300 may evaluate the transmittance of the etalonfilter 33 by a ratio of the output of the second m-PD 34 b to the outputof the first m-PD 34 a. Practical transmittance of the etalon filter 33may be specified by the specification thereof, the ratio of the twooutputs may determine the wavelength of the second CW beam L2 bycomparing this ratio with the specification of the etalon filter 33.Moreover, controlling the biases supplied to the t-LD 10 and thetemperature thereof by the first TEC 11 such that the ratio of theoutputs of the two m-PDs, 32 a and 32 b, comes closer to thetransmittance of the etalon filter 33 at a target wavelength, theemission wavelength of the t-LD 10 may coincide with the targetwavelength. An etalon filter has been known as an optical device whosetransmittance periodically varies against wavelengths. Accordingly, whenthe period of the periodic transmittance of the etalon filter matcheswith a span between nearest grids defined in the wavelength divisionmultiplexing (WDM) system, which is 100, 50, and/or 25 GHz in thespecification of the WDM system, the optical module 1 of the embodimentmay easily set the emission wavelength to be equal to one of the gridwavelengths of the WDM system.

The temperature dependence of the periodic transmittance of the etalonfilter 33 is far smaller than that of the emission wavelength of thet-LD 10. However, the present optical module 1 provides the laser unit100 and the detector unit 300 each independently providing TECs, 11 and31, because a temperature variation in a TEC in the laser unit slightlyaffects the transmittance of an etalon filter when the laser unit andthe detector unit provide a common TEC.

Also, the output of the first m-PD 34 a, which directly senses a portionof the second CW beam L2, namely, a portion of the optical beam notpassing through the etalon filter 33, may be served for controlling theoutput power of the t-LD 10. That is, by feeding the output of the firstm-PD 34 a back to the bias, particularly, the injection current into thegain regions, 12 a to 12 c, in the SG-DFB 10 b, the optical module 1 maycontrol the magnitude of the second CW beam L2 in a constant level,which may be called as the automatic power control (APC) of a t-LD 10.

Modulator Unit

FIG. 7 is a plan view of a modulator unit 200, and FIG. 8 is an explodedview of the modulator unit 200. The modulator unit 200 includes an inputunit 210 and a joint unit 220 each couple the first CW beam L1 outputfrom the laser unit 100 with the optical modulator 20. The input unit210 includes a BS 61 and a lens system 63 that are mounted on the base200 a through another carrier 210 a, which will be referred as thesecond carrier. The joint unit 220 includes a beam shifter 81 and anoptical isolator 82 on the base 200 a through another carrier 220 a,which will be referred as the third carrier. The first CW beam L1 outputfrom the laser unit 100 enters the input port 24 of the opticalmodulator 20 after the beam shifter 81 compensates a level differencebetween the optical axis of the laser unit 100 and the input port 24 ofthe optical modulator 20, and the optical isolator 82 cuts the backwardbeam from the optical modulator 20 to the t-LD 10. The BS 61 may be atype of prism mirror shown in FIG. 8 or parallel plate shown in FIG. 7each made of material substantially transparent for the first CW beamL1. The BS 61 reflects a part of the first CW beam L1, about 95%thereof, and transmits a rest 5% toward the m-PD 62 a which is mountedon the base 200 a through a PD sub-mount 62A.

Base

FIG. 9 is a plan view of the base 200 a, which may be made of MN and hasan L-shape with a cut 200 c in a coiner between a horizontal bar and avertical bar of the L-character. Referring to FIG. 8, the second TEC 21,similar to the aforementioned TECs, 11 and 31, has a rectangular planeshape with posts, 21 c and 21 d, in a corner thereof facing the rearwall 2B and the side wall 2C of the housing 2. The Peltier elementsmounted on the bottom plate 21 b of the second TEC 21 may be suppliedwith a current through the posts, 21 c and 21 d.

The base 200 a, which has a plane shape of an L-character, is mounted onthe second TEC 21 in an area closer to a corner of the L-character. Thebase 200 a has an area greater than the area of the top plate 21 a ofthe second TEC 21. That is, even when the base 200 a is mounted on thesecond TEC 21, periphery portions on the base 200 a are not overlappedwith the top plate 21 a of the second TEC 21. The base 200 a provides acut 200 c in the corner of the L-character, through which two posts, 21c and 21 d, of the second TEC 21 are exposed. The tops of the posts, 21c and 21 d, project from the base 200 a, that is, the top levels of theposts, 21 c and 21 d, are set higher than the primary surface of thebase 200 a, which enhances the productivity, or the wiring to the top ofthe posts, 21 c and 21 d.

Two areas, 200B and 200C, in the base 200 a, which correspond to endportions of respective bars of the L-shape, are not overlapped with thetop plate 21 a of the second TEC 21. That is, the two areas, 200B and200C, protrude from respective edges of the top plate 21 a of the secondTEC 21. The latter area 200C mounts the output unit 230, while, theformer area 200B mounts the input unit 210 and the joint unit 220. Thejoint unit 220 is set forward of the input unit 210.

The base 200 a has a size substantially equal to that of the opticalmodulator 20. That is, base 200 a has a lateral width substantiallyequal to a lateral width of the optical modulator 20 but narrower than alateral width of the top plate 21 a of the second TEC 21. Mounting thebase 200 a on the top plate 21 a of the second TEC 21 and the outputunit 230 on the base 200 a, the front edge of the second TEC 21 locateson a position of the second lens 73 b in the output unit 230, where thesecond lens 73 b is set apart from the optical modulator 20 comparedwith the first lens 73 a. Two m-PDs, 64 a and 64 b, are assembled on thebase 200 a. The m-PD 64 a is mounted on the side of a sub-mount 64A, andthe m-PD 64 b is mounted on the side of a sub-mount 64B. The carrier 20a is located between m-PDs, 64 a and 64 b and between the sides of thesub-mount 64A and the sub-mount 64B.

The m-PDs, 64 a and 64 b, each have optically sensitive surfaces facingthe optical modulator 20 to sense the monitor signals, M2 a and M2 d,output from the monitor ports, 25 a and 25 b, of the optical modulator20. The m-PDs, 62 a and 62 b, are assembled diagonally on respectivesides of the optical modulator 20 corresponding to the positions of themonitor ports, 25 a and 25 b.

Terminator Unit

The terminator units, 84 a and 84 b, are arranged in front sides of them-PDs, 64 a and 64 b, so as to put the optical modulator 20therebetween. FIG. 10 is a plan view of one of the terminator units 84b, and the terminator units 84 a has the same arrangement with thoseshown in FIG. 10. The terminator unit 84 a includes terminators 85 b,via-holes 85 c, and die-capacitors 85 d on a terminator carrier 84B madeof ceramics, typically, alumina (Al₂O₃). The terminator 85 b is a typeof thin film resistor formed on the ceramic carrier 84B with resistanceof 100Ω. The terminator 85 b may terminate the interconnections 45 b inthe optical modulator 20 that transmit modulation signals to the MZelements, 51M to 54M. The interconnections 45 b shown in FIG. 10 isdivided into two groups each having three interconnections andcorresponding to two MZ elements, 53M and 54M. Respective centerinterconnections in the respective groups are wired to the ground pad 85f and the respective two interconnections are wired to the signal pads85 e.

The ceramic carrier 84B in a top surface thereof providesinterconnections 85 h, while, a whole back surface thereof provides theground pattern. The ground pad 85 f provides a via-hole 85 c in a centerthereof and connected to the ground pattern of the back surface of theceramic carrier 84B. Interconnections 85 h connected to the respectiveterminators 85 b are connected to the ground pattern in the back surfaceof the carrier 84B through respective die-capacitors 85 d and thevia-hole 85 c. The interconnecting 85 h common to respective terminators85 b may be externally biased. Thus, the interconnections 45 b in theoptical modulator 20 may be terminated in the AC mode through theterminators 85 b as biased in the DC mode through the interconnections85 h. The terminator units, 84 a and 84 b, are mounted on the base 200 athrough respective carriers, 88A and 88B, provided commonly to the biasunits, 86 a and 86 b.

Bias Unit

Two bias units, 86 a and 86 b, are arranged in side by side to theterminator units, 84 a and 84 b, on the common carriers, 88A and 88B,and sandwiches the optical modulator 20 therebetween. FIG. 11 is a planview showing one of the bias units 86 b, but the other of the bias units86 a has the same arrangement with those shown in FIG. 11. The opticalmodulator 20 provides two offset electrodes, 51 j and 51 k, for theX-polarization and the 0° signal, those, 52 j and 52 k, for theX-polarization and 90° signal, those, 53 j and 53 k, for theY-polarization and 0° signal, and those, 54 j and 54 k, for theY-polarization and 90° signal. In addition, the optical modulator 20further provides the quadrature electrodes, 51 c and 52 c, for theX-polarization and those, 53 c and 54 c, for the Y-polarization. Thus,the optical modulator 20 is necessary to be supplied with twelve (12)biases. One of the bias units 86 a provides 6 biases for theX-polarization and the other of the bias units 86 b supplies 6 biasesfor the Y-polarization.

The bias unit 86 b, as shown in FIG. 11, provides six (6) die-capacitors87 a and some interconnections 87 b. The six biases are supplied torespective electrodes 45 b in the optical modulator 20 through theinterconnections 87 b and the die-capacitors 87 a. Some of theinterconnections 87 b are served for grounding the back surfaces of thedie-capacitors 87 a.

Input Unit

FIG. 12 is a plan view of the input unit 210. The input unit 210 ismounted on the base 200 a in a portion 200D, which extends from a centerportion 200A thereof, through a carrier 210 a made of AlN, which will bereferred as the second carrier. That is, the input unit 210 positions onthe base 200 a in an end of the vertical bar of the L-character.

The input unit 210 includes, in addition to the carrier 210 a, the inputlens system 63 including the first lens 63 a and the second lens 63 b,and a beam splitter (BS) 61. The first CW beam L1 generated in the laserunit 100 is bent by a right angle by the BS 61 to couple the input port24 of the optical modulator 20 through the input lens system 63.

The input unit 210, as described above, has the two-lens system 63including the first lens 63 a closer to the optical modulator 20 and thesecond lens 63 b.

FIGS. 13A and 13B compare the two-lens system (FIG. 13B) with aconventional one-lens system (FIG. 13A). The one-lens system of FIG. 13Aand the two-lens system of FIG. 13B both assume the aspheric lenses inwhich a curvature of the optical beam incoming surface is different froma curvature of the optical beam outgoing surface. Specifically, the onelens system assumes an aspheric lens 63 with an optical beam incomingsurface and an optical beam outgoing surface both having sphericalsurfaces but the curvatures thereof are different from each other,while, the first lens 63 a in the two-lens system shown in FIG. 13B hasan arrangement similar to that of the lens 63 in the one-lens system butthe second lens 63 b in the two-lens system has a plane surface for theincoming optical beam.

Also, the first lens 63 a in the two-lens system has a thicknessdifferent from the lens 63 in the one-lens system. For instance, thelens 63 in FIG. 13A has a thickness W1 of 0.84 mm, but the first lens 63a and the second lens 63 b in FIG. 13B have thicknesses, W5 and W3, of0.7 mm and 0.65 mm, respectively. A distance W2 from the beam outgoingsurface 63B of the lens 63 to the input port 24 is set to be 0.25 mm ora distance W6 from the beam outgoing surface 63 aB to the input port 24is also 0.25 mm. The distance W4 from the beam outgoing surface 63 bB ofthe second lens 63 b to the beam incoming surface 63 aA in FIG. 13B is0.5 mm. For such lens systems, focal lengths at which the opticalcoupling efficiency of the CW beams to the input port 24 of the opticalmodulator become 645 μm.

FIG. 14A to FIG. 14F show alignment tolerances of the lens in theone-lens system and those in the two-lens system, where vertical scalesare normalized with respect to the maximum coupling efficiency. FIG. 14Ashows a coupling tolerance of the lens 63 in the one-lens system for adeviation from the position at which the maximum coupling efficiency isobtained, along the x-direction, namely perpendicular to the opticalaxis of the lens. FIG. 14B also shows a coupling tolerance of the lens63 for the deviation from the position along the optical axis,z-direction. FIG. 14C and FIG. 14D show the tolerances of the couplingefficiency of the first lens 63 a in the two-lens system along the x-and z-directions, and FIGS. 14E and 14F also show the couplingtolerances for the second lens 63 b in the two-lens system along the x-and z-directions.

The lenses, 63, 63 a and 63 b, are fixed at respective positions wherethe maximum coupling efficiency against the input port 24 is realized byadhesive material typically ultraviolet curable resin. However,solidification of such resin inevitably shrinks through curing, whichcauses positional deviations of the lenses and degrades the couplingefficiency. Assuming that 20% reduction in the coupling efficiency isacceptable, the lens 63 in the one-lens system and the first lens 63 ain the two-lens system show tolerances along the x-direction of 1.04 and0.97 μm, respectively. These values are comparable to the shrinkage ofthe adhesive resin. Accordingly, in the one-lens system, even the lens63 is aligned in the position at which the maximum coupling efficiencyis realized, this maximum coupling efficiency may not secured after thesolidification of the adhesive resin, and, no means are left tocompensate the degraded coupling efficiency.

On the other hand, the second lens 63 b in the two-lens system shows thealignment tolerances far greater than those of the lens 63 in theone-lens system and the first lens 63 a. In particular, the second lens63 b shows a large tolerance, about two figures greater than that of thefirst lens 63 a, along the z-direction. Even when the second lens 63 bdeviates from the designed position by 230 μm, the degradation of thecoupling efficiency may be set within −0.5 dB. For the tolerance alongthe x-direction, the second lens 63 b shows a greater tolerance, severaltimes greater than that of the first lens 63 a, and that of the lens 63in the one-lens system. Accordingly, the two-lens system may securelyrecover or compensate by the second lens 63 b the coupling efficiencydegraded by the shrinkage of the adhesive resin for the first lens 63 a.The adhesive resin for the second lens 63 b also shrinks during thesolidification thereof. However, the shrinkage with the second lens 63 bis negligibly smaller compared with the large positional toleranceacceptable for the second lens 63 b.

The carrier 210 a further mounts the m-PD 64 via the PD sub-mount 64A,four interconnections 63 c along a side 210 b facing the joint unit 220to carry the sensed signals output from the m-PDs, 64 a and 64 b, othertwo interconnections 63 d along one side 210 c to carry another sensedsignal output from the m-PD 62 a. Two of the four interconnections 63 care for the first m-PD 64 a, and the other two interconnections 63 c arefor the second m-PD 64 b mounted in another side of the opticalmodulator 20. Wiring from the PD sub-mount 64B for the m-PD 64 b in theother side to the PD sub-mount 64A across the optical modulator 20 andfurther wiring the PD sub-mount 64A to the interconnections 63 c, thesensed signal output from the m-PD 64 b may be carried to the DCterminals 5 a in the side wall 2C. The m-PD 62 a mounted behind the BS61 may sense the magnitude of the first CW beam L1 entering the opticalmodulator 20. The BS 61 splits the CW beam L1 by a ratio of 5:95, thatis, 5% of the CW beam L1 passes the BS 61, and the rest 95% thereof isreflected by the BS 61 toward the lens system 63. The sensed signaloutput from the m-PD 62 may be carried on the interconnections 63 dprovided along the side 210 c of the carrier 210 a, and wire-bonded tothe DC terminals 5 a in the side wall 2C. Feeding the sensed signal ofthe m-PD 62 a to the bias supplied to the SOA 10 a in the t-LD 10, thefirst CW beam L1 entering the optical modulator 20 may be kept in themagnitude thereof in constant. The arrangement of the wirings thusdescribed may enable the sensed signals output from the m-PDs, 62 a to64 b, to be extracted from the DC terminals 5 a in one side wall 2C,even when the m-PD 62 b is placed in the side of the other side wall 2D.Moreover, the interconnections 63 c on the carrier 210 a may notinterfere with the optical axis of the first CW beam L1 connecting thelaser unit 100 to the BS 61.

Joint Unit

FIG. 15 is a plan view of the joint unit 220. The joint unit 220,similar to the input unit 210, provides a carrier 220 a with arectangular shape, which will be referred as the third carrier, ismounted on the area 200B of the base 200 a and upstream the input unit210 closer to the laser unit 100, where the area 200B extends from thecenter area 200A overlapping with the top plate 21 a of the second TEC21. The joint unit 220 includes a beam shifter 81, an optical isolator82, and some interconnections 220 d on the top surface of the carrier220 a. The interconnections 220 d are wired between the beam shifter 81and the optical isolator 82.

The beam shifter 81 compensates a vertical discrepancy between theoptical axis of the laser unit 100 and the input port 24 of the opticalmodulator 20. The laser unit 100 and the modulator unit 200 are mountedon respective TECs, 11 and 21, independent to each other. Thisarrangement often cause an offset between the optical axes of componentsin the laser unit 100 and those in the modulator unit 200 within a rangeof allowable tolerances in physical dimensions of those components.Also, even in the modulator unit 200, the coupling unit 220, the inputunit 210, and the optical modulator 20 are mounted on the base 200 a viarespective carriers, 20 a, 210 a, and 220 a, independent to each other.Accordingly, vertical discrepancies between optical axes of components,namely, the optical isolator 82, the BS 61, the lens system 63, and theoptical modulator 20 are often encountered. Adhesive resin to fix the BS61 and the lens system 63 on the carrier 210 a may adjust the verticaldiscrepancies of the optical axes. However, when the offset between theoptical axes of the laser unit 100 and the input port 24 of the opticalmodulator 20 becomes large, or exceeds an allowable limit, the resin inthicknesses thereof may not compensate those discrepancies in theoptical axes. The lens system 63 is impossible to lower the top level ofthe carrier 210 a, and thicker adhesive resin for the lens system 63 maydegrade the reliability of the fixation.

The beam shifter 81 of the embodiment may compensate the offset betweenthe optical axis of the laser unit 100 and that of the optical modulator20. The beam shifter 81 is a rectangular block with a beam incomingsurface and a beam outgoing surface extending in parallel to each otherand made of material transparent to the first CW beam L1. Setting thebeam shifter 81 on the carrier 220 a as vertically inclining against thetop surface of the carrier 220 a, the optical axis of the first CW beamL1 may translate vertically. The beam shifter 81 is also set on thecarrier 220 a inclined horizontally so as to prevent the first CW beamL1 back to the laser unit 100.

The interconnections 220 d are wired between the beam shifter 81 and theoptical isolator 82 between one side facing the terminator unit 84 a andthe bias unit 86 a to another side facing the side wall 2C so as toavoid the beam shifter 81. Similar to the interconnections 63 c on theinput unit 210, the interconnections 220 d on the joint unit 220 may notinterfere with the optical axis of the first CW beam L1. The terminatorunit 84 a and the bias unit 86 a are electrically connected to the DCterminals 5 a in the side wall 2C through the interconnections 220 d.Although the interconnections 220 d shown in FIG. 15 avoid the beamshifter 81, the interconnections 220 d may intersect the beam shifter81, that is, the interconnections 220 d may run beneath the beam shifter81. Because signals are carried on the interconnections 220 dsubstantially in the DC mode, the quality of those signals is notaffected by an environment of the wiring.

Output Unit

FIG. 16 is a plan view of the output unit 230. The output unit 230includes the output lens system 73 comprising two first lenses 73 a andtwo second lenses 73 b. The output lens system 73 converts two modulatedbeams, M2 b and M2 c, output from the optical modulator 20 intocollimated beams, multiplexes the collimated two beams, and outputs themultiplexed beam to the first output port 3 a as the output beam D1. Theoutput unit 230 further includes a skew adjuster 74, two opticalisolators, 75 a and 75 b, a polarization beam combiner (PBC) unit 76,and a variable optical attenuator (VOA) 77. The skew adjuster 74 maycompensate a difference of optical paths from the optical modulator 20to the PBC unit 76 for the respective modulated beams, M2 b and M2 c.The PBC unit 76 includes a reflector 76 a and a PBC element each made ofmulti-layered optical films.

One of the output lens systems 73 collimates the modulated beam M2 ctoward the first output port 3 a, while, the other of the output lenssystems 73 also collimates the other modulated beam M2 b toward themirror 76 a in the PBC unit 76. The output lens systems 73 each includethe first lens 73 a set closer to the optical modulator 20 and thesecond lens 73 b set closer to the PBC unit 76. The two modulated beams,M2 b and M2 c, are each collimated by the respective lens system 73.

One of the modulated beams M2 c is collimated by the output lens system73 and enters the PBC unit 76 as passing through the skew adjuster 74and the optical isolator 75 b. The other modulated beam M2 b is alsocollimated by the output lens system 73 and enters the PBC unit 76 aspassing through the optical isolator 75 a. The skew adjuster 74 maycompensate the optical path difference of the two modulated beams, M2 band M2 c. That is, the modulated beam M2 b comes the PBC element 76 brunning on an extra path from the mirror 76 a to the PBC element 76 bcompared with the other modulator beam M2 c that directly comes straightto the PBC element 76 b from the optical modulator 20. The skew adjuster74, by being set intermediate of the optical path for the modulated beamM2 c, may compensate the optical length of this extra path. The skewadjuster 74 of the embodiment may be a block made of materialtransparent for the first CW beam, silicon (Si) in the presentembodiment, and set slightly inclined with respect to the optical axisof the modulated beam M2 c to prevent the optical beam reflected therebyfrom coining back to the optical modulator 20.

The modulated beams, M2 b and M2 c, inherently have the polarizationreflecting that of the first CW beam entering the optical modulator L1,because the optical modulator 20 includes no components to rotate thepolarization of the incident beam. Accordingly, two modulated beams, M2b and M2 c, have the polarization identical to each other. Two opticalisolators, 75 a and 75 b, may rotate the polarization of the incidentbeams, M2 b and M2 c, independently, that is, the optical isolators, 75a and 75 b, may set a difference of 90° in the polarization between twooutgoing beams. For instance, setting a half-wave plate (λ/2-plate),which may rotate the polarization of incident beam by 90°, only in oneof the optical isolates, two modulated beams, M2 b and M2 c, output fromthe optical isolators, 75 a and 75 b, may show the polarization statusdifferent by 90° to each other. The modulated beams, M2 b and M2 c,enter the PBC element 76 b as maintaining the polarization statusthereof.

The PBC element 76 b includes multi-layered optical films and shows apeculiar property depending on the polarization of the incoming beam.For instance, the PBC element 76 b may show large reflectance,equivalently small transmittance, for the incident beam having thepolarization within the incident plane while large transmittance,equivalently small reflectance, for the incident beam with thepolarization perpendicular to the incident plane, where the incidentplane may be formed by the optical axis of the incident beam and thenormal of the incident surface of the PBC element 76 b. Setting thepolarization direction of the modulated beam M2 c in perpendicular tothe incident plane for the PBC element 76 b, but that of the othermodulated beam M2 b in parallel to the incident plane, the formermodulated beam M2 c in almost all portion thereof may transmit the PBCelement 76 b, and the latter modulated beam M2 b in almost all portionthereof may be reflected by the PBC element 76 b. Thus, the twomodulated beams, M2 b and M2 c, may be effectively multiplexed, e.g.,polarization-multiplexed, by the PBC element 76 b by rotating thepolarization of one of the modulated beam M2 b by 90° by the opticalisolator 75 a. The PBC unit 76 outputs thus multiplexed beam to the VOA77.

The two optical isolators, 75 a and 75 b, are the type of polarizationdependent isolator. By setting a magnet, not shown in figures, forinducing magnetic fields commonly to the isolators, 75 a and 75 b, theembodiment is implemented with the integrated optical isolator 75.Moreover, the description above concentrates on an arrangement whereonly the optical isolator 75 a provides the λ/2-plate in the outputthereof. However, an alternative may be applicable where one of theoptical isolators 75 a sets the crystallographic axis thereof in −22.5°but the other isolator 75 b sets the crystallographic axis in 22.5° withrespect to the polarization direction of the modulated beams, M2 b andM2 c. Then, the modulated beams, M2 b and M2 c, output from respectiveoptical isolators, 75 a and 75 b, have the respective polarizationdirections thereof perpendicular to each other.

Thus, the arrangement, where two first lenses 73 a and two second lenses73 b, the skew adjuster 74, the optical isolator 75, and the PBC unit76, are mounted on the base 200 a via the carrier 230 a made of AlN,which will be referred as the fourth carrier, may simplify the opticalalignment for those components with respect to the modulated beams, M2 band M2 c, output from the optical modulator 20. Because those opticalcomponents on the carrier 230 a inherently have dull temperaturecharacteristics, it is unnecessary to control temperatures of thosecomponents by the second TEC 21 in the modulator unit 200. Accordingly,the area 200C of the base 200 a, where the carrier 230 a is mounted, isoverhung from the area 200A overlapping with the top plate 21 a of theTEC 20 and leaves a wide space under the carrier 230 a. The opticalmodule 1 of the present invention installs two wiring substrates, 90 aand 90 b, to carry signals from the DC terminals 5 b in the side wall 2Dof the housing 2 to the laser unit 100 installed in the side of theother side wall 2C.

The reason to set the VOA 77 downstream the PBC unit is, when theoptical module 1 is installed within an optical transceiver havingfunctions to transmit an optical signal and to receiver another opticalsignal concurrently, a situation is probably encountered where only thefunction of the signal transmission is killed as leaving the function ofthe signal reception. In such a case, only the second output D2 of theoptical module 1 is required. When the biases supplied to the t-LD 10 iscut to stop the operation thereof, the second output D2 also disappears.The VOA 77 set in the path for the first optical output D1 may interruptthe operation only of the signal transmission.

When a VOA is set in upstream of the optical modulator 20, the functionto stop the signal transmission may be realized. However, thisarrangement fully suspends the input of the first CW beam L1 to theoptical modulator 20. The optical modulator 20 is necessary to adjustthe biases supplied to the offset electrodes and the quadratureelectrodes using the first CW beam L1 to adjust the phases of twooptical outputs, M2 b and M2 c. Such adjustments may be carried out forthe modulated signals, M2 a and M2 d, output from the monitor ports, 25a and 25 b, even when the modulated beams, M2 b and M2 c, are suspended.

The optical module 1 sets the m-PD 79 a in downstream of the VOA 77. Them-PD 79 a, which is mounted in a side of the PD sub-mount 79A, senses aportion of the optical output D1 split by the BS 78. The m-PD 79 a, thePD sub-mount 79A, and the BS 78 are mounted on the VOA carrier 77A,which is placed on the bottom of the housing 2 independent of thecarrier 230 a. The output of the m-PD 79 a is used for detecting thedegradation of elements integrated within the optical modulator 20 andthe excessive output power of the optical module 1.

As shown in FIG. 16, the output unit 230 is also implemented with thetwo-lens system 73 for respective modulated beams, M2 b and M2 c. Thefield pattern of the modulated beams, M2 b and M2 c, is usually deviatedfrom a true round reflecting the cross section of the waveguide in theoptical modulator 20. Such a distorted beam usually degrades thecoupling efficiency against an optical fiber with a circular fieldpattern. The two-lens system of the present optical module 1 maysuppress the reduction of the coupling efficiency between the opticalbeam with a distorted filed pattern and an optical medium with acircular cross section. In an alternative, the optical module 1 may seta beam shaper downstream the PBC unit 76 to modify the field pattern ofthe output beam D1.

FIG. 17 shows a cross section of the optical module 1, which is takenalong the optical axes of the first CW beam L1 and the second CW beamL2. Referring to FIG. 17, the carrier 220 a of the joint unit 220 doesnot interfere with the optical axis of the first CW beam L1 coining fromthe t-LD 10 to the BS 61. Referring to FIG. 15, the joint unit 220provides the interconnections 220 d that carries the biases from the DCterminals 5 a in the side wall 2C to the pads 46 a on the opticalmodulator 20 connected to the offset electrodes, 51 j to 52 k, and thequadrature electrodes, 51 c and 52 c. When the pads 46 a are directlywire-bonded to the DC terminals 5 c, not only the bonding wires lengthenbut sometimes interrupt the optical axis of the first CW beam L1. Theoptical module 1 of the embodiment avoids the optical axis by theinterconnections 220 d on the joint unit 220 a. That is, the pads 46 aon the optical modulator 20 are first wired to the ends of theinterconnections 220 d, and further wired in the other ends thereof tothe DC terminals 5 a. Thus, the optical axis of the first CW beam L1does not interfere with members except for the beam shifter 81 and theoptical isolator 82.

Also, the carrier 210 a of the input unit 210 mounts the m-PD 64 a viathe PD sub-mount 64A. The m-PD 64 a optically couples with the monitorport 25 a. The biases supplied to the offset electrodes, 51 j to 52 k,and the quadrature electrodes, 51 c and 52 c may be determined based onthe output of the m-PD 64 a. The interconnections 63 c on the carrier210 a that carries the output of the m-PD 64 a to the DC terminal 5 aalso does not interfere with the optical axis of the first CW beam L1.

The area A3 of the base 200 a mounts the terminator unit 84 a inaddition to the bias unit 86 a. The terminator unit 84 a provides fourterminators 85 b and two capacitors 85 d. The terminators 85 b terminatethe interconnections, 41 and 42, carrying the modulation signals to theMZ elements, 51M and 52M. The modulation signals provided to therespective MZ elements, 51M to 54M, have magnitudes of about 1 Vp-p. Theterminators 85 b with impedance of 50Ω for such modulation signals eachconsume the power of 20 mW. Accordingly, the optical modulator 20 of theembodiment sets the terminators externally to suppress the powerconsumption thereof. However, bonding wires from the optical modulator20 to the terminators 85 b are necessary to be short as possible, theterminator units, 84 a and 84 b, are set immediate to the opticalmodulator 20.

The area B1 of the base 200 a mounts the other m-PD 64 b via the PDsub-mount 64B for the MZ elements, 53M and 54M, and the area B2 mountsthe other terminator unit 84 b and the other bias unit 86 b, where thearrangements of those units, 84 b and 86 b, are same with thoseaforementioned units, 84 a and 86 a.

As described, the optical modulator 20 is mounted on the base 200 a, andthe base 200 a is mounted on the top plate 21 a of the second TEC 21. Anoptical modulator like the present embodiment inherently shows dulltemperature dependence of characteristics thereof. However, the opticalcoupling between the optical modulator 20, the input unit 210, the jointunit 220, and the output unit 230 may be varied depending on thetemperature, which is generally called as the tracking error.Accordingly, the present optical module 1 mounts those units, 210, 220,and 230, commonly on the base 200 a, and the base 200 a is set on thesecond TEC 21 to suppress the tracking error. However, the temperaturedependence of the optical coupling of those units, 210, 220, and 230,are far smaller than that of the t-LD 10. Accordingly, the base 200 a ofthe present embodiment mounts those units, 210, 220, and 230 on theareas, 200B and 200C, not overlapping with the TEC 21.

FIG. 18 schematically shows a cross section of the optical module 1taken along the optical axis of the first output port 3 a. The outputunit 230 is mounted on the area A3 of the base 200 a projecting from thesecond TEC 21 via the carrier 230 a, which forms a space under theoutput unit 230. The optical module 1 installs two wiring substrates, 90a and 90 b, in this space to supply the biases from the DC terminals 5Bin the side wall 2D to the t-LD 10.

FIGS. 19 and 20 are plan views of the arrangement around the wiringsubstrates, 90 a and 90 b, and FIG. 21 is a perspective view of thewiring between the wiring substrates, 90 a and 90 b, and the laser unit100. As already described, the t-LD 10 of the present embodiment isnecessary to be biased in the electrodes, 14 a to 14 e, to injectcarriers into two SOAs, 10 a and 10 d; in the heaters, 15 a and 15 b, inthe SG-DFB 10 b, and the heaters, 17 a to 17 c, in the CSG-DBR 10 c; intwo heater grounds; and in the signal ground, where total of ten (10)electrodes are necessary to be supplied with respective biases. In acase where these electrodes are biased from the DC terminals 5 aarranged in the side wall 2C closer to the laser unit 100 compared withthe other side wall 2D, the DC terminals 5 a in the number thereofoccasionally becomes insufficient when the detector unit 300 and themodulator unit 200 are also biased from the DC terminals 5 a. On theother hand, the other side wall 2D along the modulator unit 200 leavesspares of the DC terminals 5 b not connected to anywhere. Accordingly,the optical module 1 supplies the biases to the t-LD 10 from the DCterminals 5 b in the side wall 2D via the wiring substrates, 90 a and 90b.

As shown in FIGS. 20 and 21, the LD carrier 100A mounts the t-LD 10 andthe thermistor 11 f thereon. Two wires W1 extracted from the thermistor11 f are connected to the DC terminals 5 a in the closer side wall 2C.The other wires W2 are extracted to the DC terminals 5 b in the otherside wall 2D through the wiring substrates, 90 a and 90 b. The wiringsubstrate 90 a closer to the second TEC 21 has a thickness greater thanthat of the other wiring substrate 90 b because of a room to wire therespective substrates, 90 a and 90 b. That is, the wiring for the wiringsubstrate 90 a is necessary to be done in a space between the base 200 aand the first TEC 11. On the other hand, the wiring to the othersubstrate 90 b may be done in a space between the carrier 300 a of thedetector unit 300 and the lens carrier 100B, which is relatively widerthan the former space. Thus, a space relatively wider is left for thewiring to the wiring substrate 90 b.

The carrier 300 a of the detector unit 300 and the lens carrier 110B onthe base 100 a of the laser unit 100, where they sandwich the wiringsubstrate 90 b therebetween, have relatively thinner thicknesses tomount the BSs, 32 a and 32 b, and the collimating lens 110 b thereon. Onthe other hand, the other wiring substrate 90 a which locates next tothe LD carrier 100A with a thickens thereof greater than a thickness ofthe lens carrier 110B to align the level of the optical axis of the t-LD10 and that of the collimating lens 110 b with each other, which meansthat the top of the t-LD 10 is higher than the top of the lens carrier110B and that the wiring substrate 90 a is necessary to have a thicknessthereof to reduce the difference in the top level between the t-LD 10and that of the wiring substrate 90 a.

Second Embodiment

FIG. 22 shows a flow chart of a process of assembling the optical moduleof the first embodiment. Next, the process of assembling the opticalmodule 1 will be described.

S1: Assembling of Laser Unit

The process first assembles the laser unit 100 independent of theoptical module 1. The t-LD 10 and the thermistor 11 f are mounted onmetal patterns on the LD carrier 100A by a conventional die-mountprocess using eutectic solder of gold tin (AuSn). FIG. 23 is aperspective view of the t-LD 10 mounted on the LD carrier 100A. The t-LD10 is mounted on a metal pattern provided on the top of the LD carrier100A and wire-bonded from electrodes corresponding to the metal patterns100 b. After the wire-boding, the t-LD 10 may be tested in the DC mode,such as the I-L characteristic of the t-LD 10, and so on, by probing themetal patterns 100 b. When the DC test finds any failures in a t-LD 10;such t-LD 10 is extracted from the subsequent production.

S2: Assembling Modulator Unit

FIG. 24 is a perspective view of the modulator unit 200. The process ofassembling the modulator unit 200 is also carried out independent of theassembly of the optical module 1. Specifically, the optical modulator 20is mounted in a center area 200A of the base 200 a as shown in FIG. 9via the modulator carrier 20 a; then, the terminator unit 84 a and thebias unit 86 a, and the terminator unit 84 b and the bias unit 86 b aremounted in the areas, A3 and B2, of the sides of the optical modulator20 on the base 200 a, respectively. These areas, A3 and B2 put theoptical modulator 20 therebetween. The terminator units, 84 a and 84 b,solder two chip capacitors 85 d in advance to be mounted on the base 200a. The terminators 85 b, which are the type of thin film resistor, areformed concurrently with the formation of the interconnections 85 h onthe terminator units, 84 a and 84 b. Although the optical module 1 ofthe embodiment uses the chip capacitors 85 d, the optical module 1 maymount the die-capacitors on the terminator units, 84 a and 84 b. For thebias units, 86 a and 86 b, the die-capacitors 87 a are soldered on thebias units, 86 a and 86 b, in advance to be mounted on the base 200 a.The terminator unit 84 a and the bias unit 86 a, and the terminator unit84 b and the bias unit 86 b are mounted on the base 200 a via thecarriers, 88A and 88B, respectively, where these carriers, 88A and 88Bhave thicknesses such that the terminator units, 84 a and 84 b, and thebias unit, 86 a and 86 b, in respective top levels become substantiallycomparable with the top level of the optical modulator 20.

In the process S2 above, the carrier 20 a is first soldered with thebase 200 a by a eutectic solder, and the optical modulator 20 is nextsoldered on the carrier 20 a also by a eutectic solder. Subsequently,the carrier 210 a for the input unit 210, which may be referred as thesecond carrier, the carrier 220 a for the joint unit 220, which may bereferred as the third carrier, the carries, 88A and 88B, commonly forthe terminator units, 84 a and 84 b, and the bias units, 86 a and 86 b,the carrier 66A for mounting the m-PD 64 b via the PD sub-mount 64B arealso soldered in respective areas on the base 200 a. The carrier 66Amounts a thermistor 66 thereon. Accordingly, the carrier 66A may becalled as the thermistor carrier. At the process for soldering the inputcarrier 210 a on the base 200 a, a rough alignment of the carrier 210 ais carried out.

Specifically, referring to FIG. 25, the carrier 210 a includes a side210 b, which faces the side 220 c of the joint unit 220, with marks, 210e to 210 g, linearly extending inward from the edge of the carrier 210a. The center mark 210 f substantially aligns with the optical axis ofthe first CW light L1 coining from the laser unit 100 to the BS 61. Theside marks, 210 e and 210 g, have distances equal to each other.Aligning the marks, 210 e to 210 g, with the marks, 220 e to 220 g, onthe carrier 220 a of the joint unit 220, the input unit 210 may beroughly aligned with the joint unit 220 only by the visual inspection.

The optical modulator 20 also provides marks, 20 c and 20 d, along theedge 20 b facing the input unit 210. The former mark 20 c corresponds tothe input port 24, while, the latter mark 20 d indicates the monitorport 25 a. These marks, 20 c and 20 d, have a shape of an isoscelesdivided into two part by a line evenly dividing a corner constitutingthe isosceles sides. However, the shapes of those marks, 210 e to 210 g,220 e to 220 g, and 20 c to 20 d, are optional.

Using those alignment marks, the rough alignment of the input port 24 ofthe optical modulator 20 with the carrier 210 a, and that between thecarrier 210 a of the input unit 210 and the carrier 220 a of the jointunit 220 may be carried out only by the visual inspection. For thealignment of the m-PD 64 a with the monitor port 25 a, because of alarge sensitive surface of the m-PD 64 a, only the rough alignment bythe visual inspection may achieve an optical coupling efficiency betweenthe m-PD 64 a and the monitor port 25 a with practically acceptablelevel.

Referring to FIG. 26, the carrier 230 a of the output unit 230, whichmay be referred as the fourth carrier, provides in a side 230 d facingthe optical modulator 20 two marks, 230 b and 230 c. Similarly, theoptical modulator 20 provides two marks, 20 e and 20 f, in a side facingthe output unit 230. The mark 230 b in the carrier 230 a aligns with themark 20 b of the optical modulator 20 and also with the optical axis ofthe modulated beam M2 b. The mark 230 c aligns with the mark 20 e andcorresponds to the optical axis of the modulated light M2 c.

The carrier 230 a also provides three marks, 230 e to 230 g, in a side230 h facing the VOA carrier 77A. The BS carrier 78A also provides threemarks, 78 e to 78 g, in a side 78 a facing the carrier 230 a. Thesemarks, 78 e to 78 g, in the BS carrier 78A align with the marks, 230 eto 230 g, in the carrier 230 a of the output unit 230. The two modulatedbeams, M2 b and M2 c, output from the optical modulator 20 aremultiplexed as passing through the BS 78. Thus, the rough alignment ofthe carrier 230 a with the optical modulator 20 and the BS carrier 78 awith the carrier 230 a of the output unit 230 may be easily performedonly by the visual inspection of those marks.

The process of assembling the optical module 1 of the present embodimentomits fine alignments for the BS 78 and the m-PD 79 a to be mounted onthe BS carrier 78A. Only the visual inspection of those marks, 78 e to78 g, and 230 e to 230 g, for the BS 78 and the m-PD 29 a may align theoutput unit 230 with the optical modulator 20 and the BS.

After mounting those carriers, 210 a, 220 a, and 230 a on the base 200a, the pads on the optical modulator 20 are wire-bonded to theinterconnections on respective carriers. Specifically, the pads, 45 aand 45 b, on the optical modulator 20 are wire-bonded to the terminators85 b on the terminator units, 84 a and 84 b; the interconnections 85 hon the terminator units, 84 a and 84 b, are wire-bonded to theinterconnections on the carrier 220 a of the joint unit 220; the pads,46 a and 46 b, on the optical modulator 20 are also wire-bonded to thedie capacitors 87 a on the bias units, 86 a and 86 b; the die capacitors87 a are wire-bonded to the interconnections 87 b on the bias units, 86a and 86 b; and the interconnections 87 b on the bias units arewire-bonded to the interconnections 220 d on the carrier 220 a of thejoint unit 220.

The embodiment thus described, the terminator unit 84 and the bias unit86 a are commonly mounted on the carrier 88A, and the terminator unit 84b and the bias unit 86 b are also commonly mounted on the carrier 88B.However, the carriers, 88A and 88B, may be divided into two parts, oneof which mounts the terminator units, 84 a and 84 b, and the other mountthe bias units, 86 a and 86 b. Further, the terminator unit 84 a and thebias unit 86 a disposed in the side of the side wall 2C of the housingmay have a substrate common to those units, 84 a and 86 a. Similarly,the terminator unit 84 b and the bias unit 86 b arranged along the sidewall 2D may have a substrate common to each units, 84 b and 86 b.Because the bias units, 86 a and 86 b, and the terminator units, 84 aand 84 b, in portions outside of the terminators 85 b process DCsignals; respective common substrates do not degrade or affect theoperation of the optical modulator 20, rather, the assembly of the biasunits and the terminator units may be simplified.

Assembling Detector Unit

The process mounts the thermistor 31 f, two m-PDs, 34 a and 34 b, asinterposing respective PD sub-mounts, 34A and 34B, on the carrier 300 a,in the outside of the housing 2. Those components are fixed onrespective metal patterns by eutectic solder. As already described, them-PDs, 34 a and 34 b, have wide optical sensitive areas with diametersthereof greater than several scores of micron-meters; accordingly, them-PDs, 34 a and 34 b, are unnecessary to be actively aligned with thet-LD 10. The etalon filter 33 is also mounted on the carrier 300 a inthis process.

S4: Assembling Optical Module

S4 a: Installing Three TECs

FIG. 27 is a plan view showing a process of installing three TECs, 11 to31, within the housing 2. The VOA carrier 77A, that mounts the VOA 77 inadvance to the installation thereof, and two wiring boards, 90 a and 90b, are concurrently installed within the housing 2. A conventionaltechnique of the die-bonding is applied to the installation of thosedevices. As shown in FIG. 27, the bottom plates, 11 b to 21 b, of therespective TECs, 11 to 31, prepare posts in areas exposed from therespective top plates, 11 a to 31 a, to supply the driving currents tothe Peltier elements. Those posts are wire-bonded to the DC terminals, 5a and 5 b, in the respective side walls, 2C and 2D, after theinstallation of the TECs, 11 to 31.

S4 b: Mounting Laser Unit and Modulator Unit on Respective TECs

The step S4 b mounts the base 100 a of the laser unit 100, which isassembled in the step S1, and the base 200 a of the modulator unit 200,which mounts various units thereon in the step S2, on the respectiveTECs, 11 and 21.

FIG. 28 is a plan view showing the process which the laser unit 100, themodulator unit 200, and the detector unit 300 are mounted on therespective TECs, 11 to 31, in the housing 2. Optical components requiredfor the active alignment are not implemented therewith. Specifically,the LD carrier 100A that mounts the t-LD 10 by the first eutectic solderin advance to the present step is mounted on the base 100 a of the laserunit 100 by the second eutectic solder whose melting point is lower thanthat of the first eutectic solder. In the present embodiment, the firsteutectic solder is made of SnAgCuBi with a melting point of about 240°C. Concurrently with the installation of the LD carrier 100A on the base100 a, two lens carriers, 110A and HOB, are set on the base 100 a.Referring to FIG. 25 again, the LD carrier 100A provides two marks, 112e and 112 g, and the lens carrier 110A provides marks, 111 e to 111 g.Aligning the marks, 111 e to 111 g, on the lens carrier 110A with themarks, 112 e and 112 g, on the LD carrier 100A only by visualinspection, the lens carrier 110A may be roughly aligned with the LDcarrier 100A. Referring to FIG. 29, the other lens carrier 110B may bealso mounted on the base 100 a by aligning marks, 114 e to 114 g, on thelens carrier 110B with marks, 113 e to 113 g, on the LD carrier 100A inthe side opposite to that appearing in FIG. 25. The two lens carriers,110A and 110B, are mounted on the base 100 a but the collimating lenses,110 a and 110 b, are not placed on respective positions on the lenscarriers, 110A and 110B. The base 100 a thus mounting the LD carrier100A and the two lens carriers, 110A and 110B, is to be set on the TEC11.

The base 200 a of the modulator unit 200, which mounts the various unitsincluding the input unit 210 and the joint unit 220, is also fixed onthe second TEC 21 by an eutectic solder. Referring to FIG. 25 again, thecarrier 220 a of the joint unit 220 provides the marks, 221 e to 221 g,in a side 221 c facing the laser unit 100. On the other hand, the lenscarrier 110A of the laser unit 100 also provides marks, 110 e to 110 g,in a side facing the joint unit 220. Aligning these marks, 221 e to 221g, with the marks, 110 e to 110 g; the modulator unit 200 may be roughlyaligned with the laser unit 100. The rough alignment using these marksdescribed above may simplify the fine alignment subsequently carried outfor lenses and so on. Positions, where the lenses are to be mounted,provide indices on the respective carriers. However, when the respectivecarriers are largely misaligned, the fine alignment sometimes becomesunable, because even the components to be finely aligned is set on theindices, substantial optical coupling efficiency could not be obtained.The alignment process inevitably begins a step to find a position atwhich substantial coupling efficiency is realized.

S4 c: Mounting Detector Unit on TEC

The process next installs the carrier 300 a of the detector unit 300onto the third TEC 31, where the carrier 300 a assembles the thermistor31 f, two m-PDs, 34 a and 34 b, and the etalon filter 33 thereon.Referring to FIG. 29, the lens carrier 110B which is assembled on the LDcarrier 100A in the aforementioned process provides marks, 115 e to 115f, in a side opposite to that facing the LD carrier 100A. The carrier300 a of the detector unit 300 is mounted on the third TEC 31 such thatmarks, 311 e to 311 f, on the carrier 300 a are visually aligned withthe marks, 115 e to 115 g, on the lens carrier 110B. Thus, the detectorunit 300 is roughly aligned with the laser unit 100.

S5: Optical Alignment

S5 a: Alignment of Input Unit

The process finally assembles optical components that are required foractive alignment. The step S5 a first aligns the input unit 210 of themodulator unit 200 with the laser unit 100 in step S5 a(a).Specifically, the first collimating lens 110 a in the laser unit 100 isnecessary to be set in a position where an optical beam output from thefirst collimating lens 110 a becomes a collimated beam. Referring toFIG. 30, the process first sets a special tool 91 d on a position towhich the beam shifter 81 is placed as practically activating the t-LD10 to emit the dispersive light therefrom. The special tool 91 d, whichprovides two mirrors fixed in parallel to each other and making an angleof 45° with respect to the optical axis, guides the first CW light L1output from the t-LD 10 outside of the housing 2 by the paralleltranslation. Checking the collimation of the first CW light L1 by anoptical detector set apart from the housing 2, where the opticaldetector is set apart about one (1) meter from the optical module in thepresent embodiment, as sliding the first collimating lens 110 a alongthe optical axis thereof, the first collimating lens 110 a is fixed in aposition where the output beam becomes a collimated beam.

Then, removing the special tool 91 d and setting the beam shifter 81 onthe carrier 220 a of the joint unit 220, the process may compensate theoffset between the optical axis of the first CW light L1 of the laserunit 100 and that of the modulator unit 200. Referring to FIG. 31, thefirst TEC 11 mounts the t-LD 10 and the collimating lens 110 a thereon,while, the modulator unit 200 mounts the optical modulator 20 on thesecond TEC 21 through the base 200 a independent of the first TEC 11.Accordingly, the optical axis of the t-LD 10 and that of the opticalmodulator 20 are usually not aligned in the levels thereof, namely,offset each other. The optical coupling system mounted on the input unit210, that is, the BS 61 and the two-lens system 63, may compensate thisdiscrepancy of the optical axes. However, it would be hard enough tocompensate the discrepancy solely by the BS 61. A rotation angle, anelevation angle, and/or a depression angle are required to align the BS61. Moreover, it would be physically impossible for the two-lens systemto lower them beyond the top of the carrier 210 a. Also, when thelenses, 63 a and 63 b, are set apart from the carrier 210 a beyond adesigned distance, the resin that fixes the lenses, 63 a and 63 b,degrades the reliability thereof. Accordingly, the beam shifter 81 ofthe embodiment compensates the offset in the optical axes between thelaser unit 100 and the modulator unit 200. The beam shifter 81 of thepresent embodiment may be a parallel-piped block made of materialtransparent to the CW beam L1 and may offset the optical axis of theincident beam by setting the incident surface thereof inclined to theoptical axis of the incident beam.

FIG. 31 schematically shows a process of aligning the beam shifter 81 atstep S5 a(b). The process measures the level of the first CW light L1output from the collimating lens 110 a and that of the input port 24 ofthe optical modulator 20 in advance to a process of assembling the beamshifter 81. The former level may be measured concurrently with theprocess of forming the collimated beam in the output of the firstcollimating lens 110 a. From two evaluated values above described, theinclined angle of the beam shifter 81 may be estimated from thefollowing equation:

Δd=t×sin θ×(1−cos θ)/√(n ²−sin²θ)),

where Δd, t, n, and θ are the offset between two optical axes, athickness of the beam shifter 81, refractive index of the materialconstituting the beam shifter 81 and an angle to be inclined for thebeam shifter 81, respectively. Evaluating the angle θ from the equationabove, the beam shifter 81 is passively set so as to make the angle θwith respect to the carrier 210 a without any active alignment.

FIG. 32 schematically shows a process of setting the BS 61 at step S5a(c). The process first aligns the angle of the BS 61 in 45° against theside wall 2C of the housing 2 using an optical source 91 e, a powermonitor 91 m, a 3 dB coupler 91 s, and an auto-collimator 91 a.Specifically, setting the side wall 2C of the housing to be an opticalreference plane, the auto-collimator 91 a is set so as to make an angleof 45° with respect to the side wall 2C. During the preparation of theauto-collimator 91 a, the optical beam coining from and reflected to theauto-collimator 91 a passes above the housing 2. Then, the BS 61 isfirst aligned in the rotation angle thereof in the space outside of thehousing 2 such that the optical beam reflected by the back surface ofthe BS 61 and detected by the power monitor 91 m through theauto-collimator 91 a becomes a maximum. Moving the BS 61 down into thehousing 2 as keeping the angle with respect to the side wall 2C, the BS61 is next adjusted in longitudinal and lateral positions thereof. Thatis, sliding the BS 61 longitudinally along the optical axis of the beamshifter 81, namely, that of the laser unit 100, and laterally along theoptical axis of the input port 24 of the optical modulator 20, aposition of the BS 61 is found, at which the monitored beam is detectedby the m-PD 64 a and/or the m-PD 64 b through the optical modulator 20.In this step, two lenses, 63 a and 63 b, are uninstalled yet and thein-PDs, 64 a and 64 b, are practically activated. Because the lightoutput from the t-LD 10 is already collimated by the first collimatinglens 110 a, the determination of the maximum of the monitored beam, thatis, the position of the BS 61, may be accomplished.

Among optical components set between the collimating lens 110 a and theinput port 24 of the optical modulator 20, the beam shifter 81, the BS61, and the two lenses, 63 a and 63 b, may shift the optical axis. Theoptical alignment in the present embodiment, only the two lenses, 63 aand 63 b, are actively aligned in positions thereof to get the maximumcoupling efficiency. Other components, namely, the beam shifter 81 andthe BS 61, have functions to roughly align the collimated beam L1 in aposition from which the fine alignment for the two lenses, 63 a and 63b, becomes possible.

The process of aligning the first lens 63 a at step S5 a(d), places thefirst lens 63 a in a designed position but yet fixed there. Then, aspractically activating the t-LD 10 and guiding the optical beam outputfrom the first lens 63 a to the optical modulator 20. Sensing themonitored beam, M2 a or M2 d, by the m-PDs, 64 a or 64 b, the positionof the first lens 63 is evaluated at which the sensed monitored beambecomes a maximum. Because no biases are supplied to the opticalmodulator 20, two m-PDs, 64 a and 64 b, may sense the respectivemonitored beams, M2 a and M2 d. Subsequent to the evaluation of thedesired position, the first lens 63 a is fixed at a position slightlyapart from the evaluated position along the optical axis of the inputport 24. An ultraviolet curable resin used for the fixation of the firstlens 63 a usually shrinks during the curing by several micron-meters,which may misalign the position of the first lens 63 a. The second lens63 b may compensate this misalignment of the first lens 63 a.

The second lens 63 b may be aligned as sensing the monitored beam, M2 aor M2 d, through the optical modulator 20. Specifically, the second lens63 b is slid from the center of the designed position alonglongitudinally, laterally, and vertically as sensing the monitored beam,M2 a or M2 d, and is fixed by also an ultraviolet curable resin at theposition at which the sensed monitored beam, M2 a or M2 d, becomes amaximum. Although the ultraviolet resin also shrinks during the curing,which causes deviations from the desirable position determined above,the second lens 63 b has positional tolerance far greater than that ofthe first lens 63 a. The first lens 63 a has the tolerance only ofsub-micron meters, while, the second lens 63 b has the positionaltolerance thereof far greater, two or three scores greater than that ofthe first lens 63 a. Accordingly, the shrink of the ultraviolet curableresin during the curing is substantially negligible for the second lens63 b. Thus, the optical active alignment of the input unit 210 iscompleted.

Alignment of Output Unit

The process next assembles the output unit 230 of the modulator unit200, where FIG. 34 magnifies a portion of the output unit 230. Becausethe input unit 210 accompanied with the laser unit 100 and the jointunit 220 is already aligned with the optical modulator 20, the first CWlight L1 is practically input to the input port 24; two output beams, M2b and M2 d, may be independently output from the output ports, 22 a and22 b, by adjusting the biases to the offset electrodes, 51 j to 54 j and51 k to 54 k, and the quadrature electrodes, 51 c to 54 c. Setting thespecial tool 91 d at a position where the second lens 73 b is to beplaced, the first lenses 73 a is positioned such that the optical beamoutput from the first lens 73 a becomes a collimated beam. Then, thefirst lens 73 a is fixed in a position slightly closer to the opticalmodulator 20 at step S5 b(a) in FIG. 22, which is illustrated in FIG.35A. Because the first lens 73 a is set closer to the optical sources,22 a or 22 b, from the focal point, the optical beam output from thefirst lens 73 a becomes a dispersive beam.

In an alternative, the optical modulator 20 is set such that only one ofthe output ports, for instance, the output port 22 a, generates themodulated beam M2 b by adjusting the biases supplied to the electrodes,51 j to 54 j, 51 k to 54 k, and 51 c to 54 c. The first lens 73 a isaligned in a position thereof such that, as detecting the optical beamoutput from the first lens 73 a at a far point through a window set inthe first output port 3 a, and an initial position of the first lens 73a is determined such that the output beam becomes a collimated beam,where one of the focal points of the first lens 73 a in the side of theoptical modulator 20 is aligned with the output port 22 a of the opticalmodulator. Then, the first lens 73 a is fixed in a point slightly closerto the optical modulator from the initial position described above alongthe optical axis thereof. Because the PBC unit 76 is assembled on thecarrier 230 a, the output beam M2 b output from the output port 22 a,which is laterally offset from the optical axis of the first output port3 a, may be detected through the first output port 3 a as passingthrough the PBC unit 76. The other first lens 73 a optically coupledwith the output port 22 d of the optical modulator 20 may be similarlyaligned with the optical modulator 20 and fixed on the carrier 230 a.

The slight length above for the first lens 73 a is set to beapproximately 90 μm. Also, the collimator shape, or the collimate-nessof the optical beam output from the first lens 73 a may be investigatedby an arrangement illustrated in FIG. 35A, where the optical beam outputfrom the first lens 73 a is investigate by a monitor, for instance aninfrared camera, placed far from the first lens 73 a, specifically,apart by at least one meter from the first lens 73 a, and an image onthe camera may be checked as sliding the first lens 73 a on the opticalaxis thereof.

The process next aligns the second lens 73 b. Referring to FIG. 35B, thesecond lens 73 b is determined in the initial position thereof byprocedures of: first placing the second lens 73 b on the carrier 230 aand monitoring the field pattern of the optical beam output from thesecond lens 73 b by the monitor (infrared camera) placed apart from theoptical module 1, and placing the second lens 73 b at a position wherethe optical beam becomes a collimator beam. Description below assumesthat the optical path from the output 22 a to the PCB 76 is the firstpath R1 and two lenses set on the first path R1 are the first lens 73 a1 and the second lens 73 b 1, while, the other path from the output port22 b to the PCB 76 is the second path R2 and two lenses are denoted asthe first lens 73 a 2 and the second lens 73 b 2. The process todetermine the initial positions may be carried out for the respectivesecond lenses, 73 b 1 and 73 b 2. However, the second lenses, 73 b 1 and73 b 2, are not permanently fixed on the carrier 230 a.

Then, as FIG. 35C illustrates, the alignment process sets a dummy port80 on the front wall 2A of the housing 2 where the first output port 3 ais to be fixed at step S5 b(b). The dummy port 80, which emulates thecoupling unit 6 that is practically fixed on the output ports, 3 a and 3b, includes a coupling fiber 80 b and a concentrating lens 80 a thatconcentrates an optical beam entering therein onto the coupling fiber 80b. An optical beam coupled to the coupling fiber 80 b may be detectedfrom another end of the coupling fiber 80 b. The optical arrangement ofthe concentrating lens 80 a and the coupling fiber 80 b are same asthose in the coupling unit 6. After setting the dummy port 80, theprocess defines positions of the second lenses, 73 b 1 and 73 b 2, wherethe optical coupling efficiency to the dummy port 80 detected throughthe coupling fiber 80 b becomes respective maxima. Specifically, slidingthe second lens 73 b 1 along the path R1 on the carrier 230 a, theposition of the second lens 73 b is evaluated at which the optical powerdetected through the coupling fiber 80 b in the dummy port 80 becomes amaximum. Subsequently, procedures same as above described are performedfor the other second lens 73 b 2, as FIG. 36A illustrates. That is,adjusting the biases supplied to the optical modulator 20, the proceduresets the optical modulator 20 in the status where only the output beamM2 c is output from the port 22 b by eliminating the other output beamM2 b. Then, adjusting the position of the second lens 73 b 2 for theother output beam M2 c and evaluating the position at which the maximumcoupling efficiency is obtained by detecting the output power throughthe coupling fiber 80 b in the dummy port 80.

Then, two maximum coupling efficiencies are compared at step S5 b(c).The description below assumes that the second path R2 causes a superiorcoupling efficacy between the output port 22 b and the PCB 76. That is,the optical power for the second path R2 becomes greater than theoptical power for the path R1. The output beam by which a greater outputpower is obtained is called as the primary beam; that is the output beamM2 c on the second path R2 is assumed to be the primary beam, while, theother output beam showing a lesser output power is called as thesubsidiary beam, that is the output beam M2 b on the first path R1 isassumed to be the subsidiary beam. After the comparison of the opticalpower for the respective paths, R1 and R2, the second lens 73 b 1 on thesubsidiary path R1 in the present assumption is permanently fixed on thecarrier 230 a by curing ultraviolet curable resin applied to the bottomof the second lens 73 b 1.

The procedure then finely adjusts the position of the second lens 73 b 2of the primary beam such that the optical power detected through thedummy port 80 becomes equal to the optical power of the subsidiary beamalso detected through the dummy port 80, as FIG. 36B illustrates. Thesecond lens 73 b 2 for the primary beam is fixed thereat also by curingthe ultraviolet curable resin. Thus, two beams, i.e., the primary beamand the subsidiary beam, may couple with the dummy port 80 in the samecoupling coefficient, which is carried out in step S5 b(d).

When the maximum output power for the subsidiary beam exceeds a designedpower, which is primarily defined by the eye-safety for a laser beam,the second lens for the subsidiary beam is positioned such that theoutput power detected through the dummy port 80 becomes equal to thedesigned maximum and the second lens 73 b 2 for the primary beam is alsopositioned such that the output power detected through the dummy port 80becomes equal to the designed maximum.

Finally, at step S5 b(e), removing the dummy port 80 from the outputport 3 a and setting the coupling unit 6 onto the first output port 3 a,the alignment of the coupling unit 6 may be carried out as follows: thatis, releasing the biases supplied to the optical modulator 20, the twobeams, M2 b and M2 c, output from the output ports, 22 a and 22 b,couple the coupling unit 6. The coupling unit 6 is aligned such that theoutput power detected through the optical fiber in the coupling unit 6becomes a maximum. The coupling unit 6 has a function to move theoptical fiber in a plane perpendicular to the optical axis thereof andin parallel to the optical axis. Accordingly, moving the optical fiberrelative to the concentrating lens 3 c in the coupling unit 6, themaximum coupling efficiency may be evaluated.

In an alternative, similar to the modified alignment procedures for thesecond lenses 73 b described above, only one of the output beams, M2 bor M2 c, is coupled with the coupling unit 6 by adjusting the biasessupplied to the optical modulator 20, and the position of the couplingfiber relative to the concentrating lens in the coupling unit 6 isaligned such that the output power detected through the optical fiberbecomes equal to that obtained in the alignment process for the secondlens 73 b. That is, supplying the biases to the optical modulator 20such that only the optical beam M2 b is output from the output port 22a, aligning the first output port 3 a with the optical beam M2 b bysliding the first output port 3 a on the front wall 2A, which is the XYalignment in a plane perpendicular to the optical axis, and adjusting adistance between the lens 3 c in the coupling unit 6 and the externaloptical fiber set in the coupling unit 6. Those procedures are iterateduntil the coupling efficiency between the external optical fiber and theoutput port 22 a of the optical modulator 20 becomes a maximum. Thecoupling unit 6 has a function to slide the external optical fiber inparallel to the optical axis. When the coupling unit 6 is once alignedfor the one of the output beams, M2 b and M2 c; the other of the outputbeams, M2 b and M2 c, may be automatically aligned because the secondlens 73 b for the other output beam is aligned such that the outputpower detected through the coupling fiber is equal to the one for theother output beam.

The reason why the second lenses 73 b are independently adjusted in thepositions thereof such that the output power detected through thecoupling unit 6 becomes equal to each other is that the two outputbeams, M2 b and M2 c, have respective polarizations perpendicular toeach other and each containing transmitting information of 0° and 90°independent to each other. Accordingly, when the output power of the twobeams, M2 b and M2 c, show a large difference, the error rate containedwithin the transmission information drastically increases.

The optical modulator 20 causes various optical losses in the waveguidesimplemented therein, which makes the output beams, M2 b and M2 c, in theoptical power thereof different from each other. Moreover, the first andsecond lenses, 73 a and 73 b, the PCB 76, the skew adjustor 74, and theoptical isolates, 75 a and 75 b, which are independently placed in therespective optical paths, R1 and R2, from the optical modulator 20 tothe first output port 3 a, possibly and independently vary the opticalcoupling efficiency between the optical modulator 20 and the firstoutput port 3 a.

The optical module 1 of the present embodiment provides the three lenssystem in the output unit 230; that is, three lenses, 73 a, 73 b, and 3c, are interposed between the output ports, 22 a and 22 b, of theoptical modulator and the optical fiber in the coupling unit 6. Thefirst lens 73 a further diffuses the output beam, M2 b or M2 c, which isoriginally diffusing beam. The second lens 73 b collimates this diffusedoptical beam output from the first lens 73 a in the initial positionthereof. The third lens 3 c in the coupling unit 6 concentrates theoutput beam, M2 b or M2 c, onto the optical fiber. In the presentembodiment, the first lens has a magnification ratio of 4; while, thesecond lens 73 b and the third lens 3 c has the magnification ratioof 1. The magnification ratio of 4 of the first lens 73 a is ideal forthe mode filed diameter of the output ports, 22 a and 22 b, of theoptical modulator 20.

Also, because the first lens 73 a shows the highest tolerance for theoptical coupling efficiency, the first lens 73 a is first aligned thenthe second lens 73 b and the third lens 3 c are aligned. Accordingly,the second lens 73 b with the unity magnification ratio may have afunction to compensate the coupling efficiency. For instance, in anarrangement where only the first lens 73 a without the second lens 73 bcollimates the diffused optical beam form the optical modulator 20, thefirst lens 73 a is inevitable to have a positional tolerance of lessthan 0.3 μm; while, a combined system with the first and second lenses,73 a and 73 b, requests the tolerance in the second lens 73 b to be lessthan 1.5 μm in order to get the designed coupling efficiency.

The first and second lenses, 73 a and 73 b, are permanently fixed to thecarrier 230 a by the ultraviolet curable resin. Ultraviolet curableresin generally shrinks during the cure by about 1 μm, which shifts theonce determined position of the first lens 73 a and resultantly degradesthe optical coupling efficiency. The second lens 73 b also shifts theposition thereof during the cure of the resin. However, because thesecond lens 73 b has the positional tolerance of 1.5 μm or less, thecure of the resin substantially cause no degradation in the couplingefficiency.

The second lens 73 b as described above shifts the position thereofalong the optical axis thereof to adjust the coupling efficiency betweenthe optical modulator 20 as the optical source and the optical fiber asthe target. Two directions are available for shifting the second lens 73b; that is, closer to the first lens 73 a or apart therefrom.

When the second lens 73 b moves closer to the first lens 73 a, theoptical beam output from the second lens 73 b becomes a diffused beam,which means that the coupling efficiency monotonically decreases asincreasing a shift distance. Accordingly, the YAG laser welding of thefirst output port 3 a to the housing 2, and/or that of the coupling unit6 with the first output port 3 a may be stably carried out. The YAGlaser welding inevitably causes miss-alignment or degrades the opticalcoupling efficiency between the members. However, the second lens 73 bis to be fixed at the position taking into the miss-alignment or thedegradation, the fixation of the first output port 3 a or the couplingunit 6 may be stably carried out.

On the other hand, the optical beam output from the second lens 73 bbecomes a concentrated beam when the second lens 73 b is set apart fromthe first lens 73 a from the initial position. Depending on the shiftdistance of the second lens 73 b, the optical beam output from thesecond lens 73 b is possibly focused before the first output port 3 a.That is, the coupling efficiency first increases, gives a maximum at theposition where the optical beam output from the second lens 73 b becomesa collimator beam, and then decreases. Accordingly, the couplingefficiency between the optical modulator 20 and the first output port 3a after the YAG laser welding of the first output port 3 a becomesinstable. However, because of the optical beam output from the secondlens 73 b becomes a concentrated beam, which means that the filed sizeof the optical beam output from the second lens 73 b at the opticalfiber becomes smaller, the coupling efficiency in a value itselfincreases compared with the case where the second lens 73 b is setcloser to the first lens 73 a.

S5 c: Alignment of Detector Unit

Before the alignment of the detector unit 300, the process first alignsthe second collimating lens 110 b mounted on the base 100 a of the laserunit 100 through the lens carrier 110B. The procedure first activatesthe t-LD 10 and sets the special tool 91 d, which is used in thealignment of the other collimating lens 110 a, at a position where thefirst BS 32 a is to be placed. The tool 91 d carries the second CW lightL2 output from the back facet 10B of the t-LD 10 out of the housing 2.Similar to the alignment of the first collimating lens 110 a, asmonitoring the second CW light L2 at a point apart from the housing 2,and the process aligns the second collimating lens 110 b in the pointwhere the monitored second CW light L2 becomes a collimated beam.Finally, the second collimating lens 110 b is fixed thereat by curingultraviolet curable resin.

Then, the process aligns two BSs, 32 a and 32 b. First, as monitoringthe second CW light L2 by the first m-PD 34 a, the first BS 32 a is slidfrom a designed position along a direction in parallel to the opticalaxis of the second CW light L2 output from the second collimating lens110 b. The first BS 32 a is fixed at the position, slightly apart from atemporal position along the optical axis of the second CW light L2, atwhich the second CW light L2 monitored by the first m-PD 34 a becomes amaximum. The reason why the first BS 32 a is slightly slid is that thesecond CW light L2 reflected by the first BS 32 a and entering thesecond m-PD 34 b is refracted by the second BS 32 b. The m-PD 34 a isset at a position slightly offset from the optical axis of the second CWlight L2 because the second CW light L2 passing through the first BS 32a and the etalon filter 33 is refracted thereby. During the alignment ofthe first BS 32 above, the process does not rotate the BS 32 because thesecond CW light L2 is converted into a collimated beam having arelatively large field diameter. The second BS 32 b is aligned asfollows: the process first sets a dummy port, which has the samearrangement with that of the aforementioned dummy port utilized in thealignment process for the output unit 230 of the modulator unit 200, onthe second output port 3 b of the housing 2. The second BS 32 b isaligned such that the optical beam reflected by the second BS 32 b anddetected through the coupling fiber in the dummy port becomes a maximum.

The optical module 1 may replace the BSs, 32 a and 32 b, of the parallelplate type with those of the prism type. A BS of the prism type stickstwo optical prisms and has a cubic plane shape. The optical alignment ofthe BS of the prism type may be accomplished by the same procedures withthose above described for the parallel plate type. That is, withoutperforming the rotational alignment of the prism BS, the first andsecond BSs are aligned as sliding parallel and perpendicular to theoptical axis of the second CW light L2 output from the secondcollimating lens 110 b to find respective positions at which the opticalpower detected through the dummy port becomes a maximum. ABS with theprism type inherently has a medium split ratio of about 10:90; that is,10% of the incident beam may transmits the BS, and the rest 90% thereofmay be reflected. Accordingly, the optical output power available at thesecond output port 3 b is reduced to 80% of the optical beam just outputfrom the t-LD 10. On the other hand, a BS with the parallel plate typeshows a split ratio of about 5:95, 5% of the incident beam transmits butthe rest 95% is reflected. Accordingly, the optical output poweravailable at the second output port 3 b becomes 90% of that of theoptical beam just output from the back face 10B of the t-LD 10, which isabout 10% greater than that available for the BSs for the prism type.The dummy port set on the second output port 3 b is replaced by thecoupling unit having the arrangements same with those of the couplingunit as aligning the coupling unit on the second output port 3 b so asto recover the optical coupling efficiency between the second BS 32 bwith the coupling port.

S6: RF Wiring

Finally, the process of assembling the optical module 1 performs thewiring from the RF terminals 4 in the rear wall 2B to the signal pads,41 to 44, on the optical modulator 20. However, the wiring for the RFpads, 41 to 44, may be carried out concurrently with the wiring for theDC terminals, 5 a and 5 b. Ceiling the housing 2, the process ofassembling the optical module 1 is completed.

Modification

The process thus described has an order to assemble respective units,100 to 300, from the laser unit 100, the input unit 210, the output unit230, and the detector unit 300. However, the process is not restrictedto this order. The alignment of the detector unit 300 may be carried outjust after the alignment of the laser unit 100 before the process ofaligning the modulator unit 200. Only the limited order is that thealignment of the input unit 210 is necessary to be done before thealignment of the output unit 230, because the latter alignment uses theoptical beams, M2 b and M2 c, output from the optical modulator 20, andthese beams, M2 b and M2 c, derive from the first CW light provided fromthe input unit 210.

The optical module 1, as described, installs the laser unit 100, themodulator unit 200, and the detector unit 300 within one housing 2,which results in a complex arrangement within the housing 2. However, anoptical coherent transceiver implementing the optical module 1 of theinvention may simplify the arrangement thereof. Such a coherent opticaltransceiver is at least unnecessary to install an optical sourceindependently. Also, the optical alignment process between the unitsbecomes unnecessary when the coherent optical transceiver installs theoptical module 1 of the invention.

The optical module 1 thus described provides TECs, 11 to 31, independentfor the laser unit 100, the modulator unit 200, and the detector unit300. Accordingly, the respective units, 100 to 300, may be preciselycontrolled in temperatures thereof depending on calorific amounts ofrespective units, 100 to 300. The emission wavelength of the t-LD 10 maybe precisely controlled independent of the temperatures of the opticalmodulator 20 and that of the detector unit 300. The optical modulator 20may be optionally controlled in the operation thereof. The detector unit300 may precisely determine the emission wavelength of the t-LD 10.

The optical alignment of the collimating lenses, 110 a and 110 b,utilizes the special tool 91 d that takes the optical beams output fromthe t-LD 10 out of the housing 2, which enables to determine thepositions of the collimating lenses at which the optical beams outputfrom the respective lenses, 110 a and 110 b, become collimated beams.Also, the input unit 210 provides the two-lens system to couple thefirst CW light L1 with the input port 24 of the optical modulator 20.The two-lens system may compensate the deviation inherently causedduring the solidification of the ultraviolet curable resin.

The optical modulator 20 of the embodiment provides the monitor ports,25 a and 25 b, that output the monitored beams, M2 a and M2 d,respectively, which are split from the output beams, M2 b and M2 c.Accordingly, the monitored beams, M2 a and M2 d, may be served for theactive alignment of the optical components in the input unit 210.

In the foregoing detailed description, the method and module of thepresent invention have been described with reference to specificexemplary embodiments thereof. It will, however, be evident that variousmodifications and changes may be made thereto without departing from thebroader spirit and scope of the present invention. The presentspecification and figures are accordingly to be regarded as illustrativerather than restrictive.

What is claimed is:
 1. A process of assembling an optical module thatincludes an optical modulator, a beam combiner, and an output port, theoptical modulator having a type of Mach-Zehnder modulator providing afirst port that outputs a first beam and a second port that output asecond beam, the first port being coupled with the output port through afirst path that implements a first lens and a second lens thereon, thesecond port being coupled with the output port through a second paththat implements a first lens and a second lens thereon, the beamcombiner combining the first beam with the second beam and generating acombined beam to the output port, the first lenses on the first path andthe second path being placed closer to the first port and the secondport compared with the second lenses on the first path and the secondpath, respectively, the method comprising steps of: moving the firstlenses on the first path and the second path closer to the first portand the second port by a preset distance from positions at which thefirst lenses on the first path and the second path convert the firstbeam and the second beam diffusively output from the first port and thesecond port into respective collimated beams; placing the second lensesat positions where the first beam and the second beam output from thefirst lenses couple with the output port with respective maximumcoupling efficiencies; determining one of the second lenses that showsthe maximum coupling efficiency greater than the maximum couplingefficiency shown by another of the second lenses; and equalizing themaximum coupling efficiencies by moving the one of the second lensesthat shows the greater maximum coupling efficiency.
 2. The process ofclaim 1, further including a step of, after moving the first lenses butbefore the placing second lenses, preparing a dummy port in a positionwhere the output port is to be set, wherein the step of placing thesecond lenses includes a step of detecting the first beam and the secondbeam output from the second lenses through the dummy port.
 3. Theprocess of claim 1, wherein the step of equalizing the maximum couplingefficiencies includes a step of moving the one of the second lenses thatshows the greater maximum coupling efficiency closer to the first lenscorresponding to the one of the second lenses.
 4. The process of claim1, wherein the optical modulator further includes a variable opticalattenuator (VOA) between the beam combiner and the output port, andwherein the step of equalizing the maximum coupling efficienciesincludes a step of moving the one of the second lenses that shows thegreater maximum coupling efficiency so as to be apart from the firstlens corresponding to the one of the second lenses.
 5. The process ofclaim 1, wherein the step of moving the first lenses includes steps of:placing the first lenses at the positions where the first lenses convertthe first beam and the second beam diffusively output from the firstport and the second port into the respective collimated beams, andslightly moving the first lenses toward the first port and the secondport, respectively, wherein the first lens and the second lens outputthe first beam and the second beam diffusively.
 6. The process of claim5, wherein the step of moving the first lenses includes a step ofdetecting the first beam and the second beam by a monitor set apartenough from the first lenses.
 7. The process of claim 5, wherein thestep of slightly moving the first lenses includes a step of moving thefirst lenses by around 90 μm from the positions at which the firstlenses convert the first beam and the second bema into the respectivecollimated beams.
 8. The process of claim 5, wherein the first lenseshave a magnification ratio of
 4. 9. The process of claim 5, wherein thestep of placing the first lenses includes a step of placing the monitorat least one meter apart from the optical modulator.
 10. The process ofclaim 8, wherein the second lenses have a magnification ratio of unity.11. The process of claim 1, wherein the step of placing the secondlenses includes a step of: placing the second lenses at positions wherethe second lenses convert the first beam and the second beam output fromthe first lenses into respective collimated beams; preparing a dummyport in a position where the output port is to be set; and moving thesecond lenses from the positions where the first beam and the secondbeam output from the second lenses become respective collimated beamssuch that the first port and the second port couple with the dummy portat the respective maximum coupling efficiencies.
 12. The process ofclaim 1, wherein the step of placing the second lenses includes stepsof: biasing the optical modulator such that only one of the first beamand the second beam is output from the first port and the second port,respectively; and placing the one of the second lenses corresponding tothe one of the first beam and the second beam currently output from theoptical modulator.
 13. The process of claim 1, further including a stepof, after the step of equalizing the maximum coupling efficiencies,replacing the dummy port with the output port; and aligning the outputport with one of the first port and the second port of the opticalmodulator.